feat(observability): query gateway core + dashboard shell

[Ma77Ball]

What changes were proposed in this PR?

Introduces the tenant-scoped read path the dashboard queries, plus the Angular shell that hosts the per-signal panels. This PR warrants the closest review: it enforces tenancy and rate limiting and is the first user-visible surface.
Backend:

• BackendClient: HTTP client to the telemetry backends.
• ScopeResolver: derives the caller's tenant scope and constrains every query to it.
• RateLimiter, AuditLogger, GatewayContext: per-request rate limiting, audit logging, and shared request context.
• dtos.scala: typed request objects with validators (time window, page size, free text, service name).
• ObservabilityResources with the /observability/health endpoint, registered in TexeraWebApplication.
• RequestContextMdcFilter and UserContextMdcFilter: inject request and user context into the logging MDC.
• ObservabilityGatewayConfig and its configuration file.
  Frontend:
• Observability dashboard page, route, and navigation entry, plus observability.service, observability.types, and the traces-pivot.service.
• Health gating: each tab is guarded by the per-signal reachability check; an unreachable signal renders an explicit state rather than a broken panel. Signal panels follow in PR5 through PR8.

Any related issues, documentation, or discussions?

Closes: #5370
Part of #4070. Stacked on #5377.

How was this PR tested?

• Backend specs for the gateway core, DTO validation, scope resolver, rate limiter, and MDC filters; sbt scalafmtCheckAll passes.
• Frontend component and service specs; prettier-eslint and eslint pass.
• Compile and the full test suites run in this PR's CI.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.8 in compliance with ASF

[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.92350% with 385 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.22%. Comparing base (5869492) to head (d93fa83).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...apache/texera/web/observability/gateway/dtos.scala	42.93%	103 Missing and 6 partials ⚠️
...ala/org/apache/texera/observability/OtelInit.scala	63.30%	43 Missing and 8 partials ⚠️
...texera/web/observability/gateway/AuditLogger.scala	0.00%	41 Missing ⚠️
...observability/gateway/ObservabilityResources.scala	0.00%	38 Missing ⚠️
...xera/web/observability/gateway/ScopeResolver.scala	19.44%	27 Missing and 2 partials ⚠️
...era/web/observability/gateway/GatewayContext.scala	0.00%	19 Missing ⚠️
...a/org/apache/texera/web/TexeraWebApplication.scala	0.00%	14 Missing ⚠️
...org/apache/texera/observability/TexeraTracer.scala	0.00%	14 Missing ⚠️
...xera/web/observability/gateway/BackendClient.scala	72.91%	5 Missing and 8 partials ⚠️
...rg/apache/texera/observability/TexeraMetrics.scala	84.93%	4 Missing and 7 partials ⚠️
... and 11 more

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5378      +/-   ##
============================================
+ Coverage     53.17%   53.22%   +0.04%     
- Complexity     2660     2697      +37     
============================================
  Files          1094     1117      +23     
  Lines         42363    43256     +893     
  Branches       4556     4717     +161     
============================================
+ Hits          22528    23022     +494     
- Misses        18507    18858     +351     
- Partials       1328     1376      +48     

Flag	Coverage Δ		*Carryforward flag
access-control-service	70.44% <ø> (ø)
agent-service	34.36% <ø> (ø)		Carriedforward from 263093a
amber	53.52% <53.72%> (-0.02%)	⬇️
computing-unit-managing-service	1.65% <ø> (ø)
config-service	56.71% <ø> (ø)
file-service	57.06% <ø> (ø)
frontend	48.21% <93.75%> (+0.25%)	⬆️
pyamber	89.84% <ø> (-0.29%)	⬇️	Carriedforward from 263093a
python	90.80% <ø> (ø)		Carriedforward from 263093a
workflow-compiling-service	58.69% <ø> (ø)

*This pull request uses carry forward flags. Click here to find out more.

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[github-actions]

⚠️ Benchmark changes need a look

🟢 2 better · 🔴 5 worse · ⚪ 8 noise (<±5%) · 0 without baseline

Compared against main 5869492 benchmarked on this same runner, so the delta is largely free of cross-runner hardware noise. The "7d avg" column still reflects the gh-pages dashboard. Treat <±5% as noise unless repeated.

Dashboard · Run

	config	throughput	MB/s	latency	max Δ latest / 7d
🔴	bs=10 sw=10 sl=64	387	0.236	25,044/38,420/38,420 us	🔴 +13.1% / 🔴 +9.8%
🟢	bs=100 sw=10 sl=64	967	0.59	102,719/118,095/118,095 us	🟢 -10.6% / 🟢 -15.5%
⚪	bs=1000 sw=10 sl=64	1,111	0.678	900,949/944,001/944,001 us	⚪ within ±5% / 🟢 -7.7%

Baseline details

Latest main 5869492 from same runner

config	metric	PR	latest main	7d avg	Δ latest	Δ 7d
bs=10 sw=10 sl=64	throughput	387 tuples/sec	417 tuples/sec	410.82 tuples/sec	-7.2%	-5.8%
bs=10 sw=10 sl=64	MB/s	0.236 MB/s	0.254 MB/s	0.251 MB/s	-7.1%	-5.9%
bs=10 sw=10 sl=64	p50	25,044 us	22,947 us	23,785 us	+9.1%	+5.3%
bs=10 sw=10 sl=64	p95	38,420 us	33,956 us	34,980 us	+13.1%	+9.8%
bs=10 sw=10 sl=64	p99	38,420 us	33,956 us	34,980 us	+13.1%	+9.8%
bs=100 sw=10 sl=64	throughput	967 tuples/sec	954 tuples/sec	891.94 tuples/sec	+1.4%	+8.4%
bs=100 sw=10 sl=64	MB/s	0.59 MB/s	0.582 MB/s	0.544 MB/s	+1.4%	+8.4%
bs=100 sw=10 sl=64	p50	102,719 us	103,873 us	112,277 us	-1.1%	-8.5%
bs=100 sw=10 sl=64	p95	118,095 us	132,080 us	139,802 us	-10.6%	-15.5%
bs=100 sw=10 sl=64	p99	118,095 us	132,080 us	139,802 us	-10.6%	-15.5%
bs=1000 sw=10 sl=64	throughput	1,111 tuples/sec	1,118 tuples/sec	1,041 tuples/sec	-0.6%	+6.7%
bs=1000 sw=10 sl=64	MB/s	0.678 MB/s	0.683 MB/s	0.635 MB/s	-0.7%	+6.7%
bs=1000 sw=10 sl=64	p50	900,949 us	899,927 us	972,714 us	+0.1%	-7.4%
bs=1000 sw=10 sl=64	p95	944,001 us	930,096 us	1,023,057 us	+1.5%	-7.7%
bs=1000 sw=10 sl=64	p99	944,001 us	930,096 us	1,023,057 us	+1.5%	-7.7%

Raw CSV

config_idx,batch_size,schema_width,string_len,num_batches,total_ms,total_tuples,total_bytes,tuples_per_sec,mb_per_sec,lat_p50_us,lat_p95_us,lat_p99_us
0,10,10,64,20,516.21,200,128000,387,0.236,25044.44,38419.84,38419.84
1,100,10,64,20,2067.54,2000,1280000,967,0.590,102719.18,118094.86,118094.86
2,1000,10,64,20,17994.38,20000,12800000,1111,0.678,900948.57,944000.53,944000.53