refactor(frontend): preserve config source in GuiConfigService instead of one flat map

[Ma77Ball]

Stacked on #5545 (adds the /config/amber endpoint and the amber config source this PR namespaces). Base diff includes #5545 until it merges; review the last commit only, or after #5545 lands.

What changes were proposed in this PR?

• Changed GuiConfigService to store each endpoint's payload under its own key in a configBySource map (preLogin, gui, amber, userSystem) instead of spreading them all into one flat object, so the origin of every config value is preserved (addresses the review comment on refactor(config): unify default-data-transfer-batch-size into one config key #5545).
• Derived the existing flat env getter from configBySource on read, keeping its shape identical so the ~50 existing guiConfigService.env.<flag> call sites are untouched.
• Memoized env so it returns a stable object reference (rebuilt only when a source loads). This preserves the two-way [(ngModel)] write-through at menu.component.html, which only persists when the same object identity is held across change-detection reads.
• Added a source(name) accessor that returns one endpoint's payload in isolation.
• Added unit tests for: source isolation (each source retrievable alone, no key bleed across sources, empty object for an unloaded source), env reference stability (stable across reads, rebuilt after a new source loads, in-memory write discarded on reload), and merge precedence (later source wins on a shared key).

Any related issues, documentation, discussions?

Closes: #5669

How was this PR tested?

• Run cd frontend && npx ng test --include="src/app/common/service/gui-config.service.spec.ts" and expect 15 passed (the 8 existing merge/orchestration tests plus 7 new tests covering source isolation, env reference stability, and merge precedence).
• Confirm the merged view is unchanged: service.env.defaultDataTransferBatchSize still resolves after post-login load, while service.source("amber") returns only the amber payload and service.source("gui") does not contain defaultDataTransferBatchSize.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.95%. Comparing base (dfa0434) to head (036179f).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5673      +/-   ##
============================================
+ Coverage     52.91%   52.95%   +0.03%     
  Complexity     2626     2626              
============================================
  Files          1090     1090              
  Lines         42188    42243      +55     
  Branches       4531     4533       +2     
============================================
+ Hits          22324    22368      +44     
- Misses        18555    18563       +8     
- Partials       1309     1312       +3     

Flag	Coverage Δ		*Carryforward flag
access-control-service	70.91% <ø> (ø)
agent-service	34.36% <ø> (ø)		Carriedforward from 7ae9b35
amber	53.00% <ø> (-0.02%)	⬇️
computing-unit-managing-service	1.65% <ø> (ø)
config-service	57.35% <100.00%> (+0.63%)	⬆️
file-service	57.06% <ø> (ø)
frontend	47.93% <100.00%> (-0.01%)	⬇️
pyamber	90.78% <ø> (+0.05%)	⬆️	Carriedforward from 7ae9b35
python	90.73% <ø> (ø)		Carriedforward from 7ae9b35
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 · 🔴 3 worse · ⚪ 10 noise (<±5%) · 0 without baseline

Compared against main dfa0434 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	433	0.264	22,946/28,301/28,301 us	🔴 +12.3% / 🟢 -19.1%
⚪	bs=100 sw=10 sl=64	938	0.573	105,454/133,418/133,418 us	⚪ within ±5% / 🟢 -6.1%
⚪	bs=1000 sw=10 sl=64	1,128	0.688	894,484/947,677/947,677 us	⚪ within ±5% / 🟢 +8.4%

Baseline details

Latest main dfa0434 from same runner

config	metric	PR	latest main	7d avg	Δ latest	Δ 7d
bs=10 sw=10 sl=64	throughput	433 tuples/sec	461 tuples/sec	410.82 tuples/sec	-6.1%	+5.4%
bs=10 sw=10 sl=64	MB/s	0.264 MB/s	0.281 MB/s	0.251 MB/s	-6.0%	+5.3%
bs=10 sw=10 sl=64	p50	22,946 us	20,430 us	23,785 us	+12.3%	-3.5%
bs=10 sw=10 sl=64	p95	28,301 us	31,414 us	34,980 us	-9.9%	-19.1%
bs=10 sw=10 sl=64	p99	28,301 us	31,414 us	34,980 us	-9.9%	-19.1%
bs=100 sw=10 sl=64	throughput	938 tuples/sec	954 tuples/sec	891.94 tuples/sec	-1.7%	+5.2%
bs=100 sw=10 sl=64	MB/s	0.573 MB/s	0.582 MB/s	0.544 MB/s	-1.5%	+5.3%
bs=100 sw=10 sl=64	p50	105,454 us	104,196 us	112,277 us	+1.2%	-6.1%
bs=100 sw=10 sl=64	p95	133,418 us	134,840 us	139,802 us	-1.1%	-4.6%
bs=100 sw=10 sl=64	p99	133,418 us	134,840 us	139,802 us	-1.1%	-4.6%
bs=1000 sw=10 sl=64	throughput	1,128 tuples/sec	1,099 tuples/sec	1,041 tuples/sec	+2.6%	+8.4%
bs=1000 sw=10 sl=64	MB/s	0.688 MB/s	0.671 MB/s	0.635 MB/s	+2.5%	+8.3%
bs=1000 sw=10 sl=64	p50	894,484 us	917,226 us	972,714 us	-2.5%	-8.0%
bs=1000 sw=10 sl=64	p95	947,677 us	955,629 us	1,023,057 us	-0.8%	-7.4%
bs=1000 sw=10 sl=64	p99	947,677 us	955,629 us	1,023,057 us	-0.8%	-7.4%

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
0,10,10,64,20,461.69,200,128000,433,0.264,22946.39,28301.25,28301.25
1,100,10,64,20,2131.91,2000,1280000,938,0.573,105453.55,133418.07,133418.07
2,1000,10,64,20,17730.65,20000,12800000,1128,0.688,894483.97,947677.09,947677.09