[config/syslog]: Resolve per-ASIC-only features to their container on single-ASIC platforms

[nsoma-cisco]

What I did

Fix config syslog rate-limit-feature enable/disable <svc> being a silent no-op on single-ASIC platforms for features whose FEATURE entry has has_global_scope=False and has_per_asic_scope=True (e.g. syncd, gbsyncd, and swss on some platforms).

How I did it

• Root cause: config/syslog.py::get_feature_names_to_proceed only enumerated per-ASIC container names inside an if multi_asic.is_multi_asic(): guard. On a single-ASIC DUT that branch was skipped, the feature list came back empty, and the outer for loop in enable_rate_limit_feature / disable_rate_limit_feature printed nothing and did nothing.
• Fix: on a single-ASIC platform a feature has exactly one container whose name equals the service name regardless of which scope flag is set, so both scopes are now treated symmetrically in that branch. Multi-ASIC behavior is preserved byte-for-byte.
• Hardened the flag lookups with .get('flag', '') to avoid a KeyError when a flag is absent.
• Safety net: enable/disable now emit an explicit "No container resolved … nothing to enable/disable" message when the resolved list is empty, so a future similar misconfiguration cannot silently no-op again.

Behavior matrix after the fix

FEATURE flags	Single-ASIC → containers	Multi-ASIC (N ASICs) → containers
global=True, per_asic=False	[svc]	[svc]
global=False, per_asic=True	[svc] (was [] — the bug)	[svc0, …, svc{N-1}]
global=True, per_asic=True	[svc] (was [] when looked up by name)	[svc, svc0, …, svc{N-1}]
global=False, per_asic=False	[] + "nothing to enable" message	[] + "nothing to enable" message

How to verify it

Unit tests:

```bash
python3 -m pytest tests/syslog_test.py tests/syslog_multi_asic_test.py -x -q
```

• New `tests/syslog_test.py::TestSyslog::test_enable_rate_limit_feature_single_asic_per_asic_scope` asserts that `docker cp .../containercfgd.conf` and `supervisorctl start containercfgd` are invoked for a single-ASIC `syncd`-like feature (`has_global_scope=False, has_per_asic_scope=True`), and the symmetric `stop` + `rm -f` on disable. Would have failed against the pre-fix code.
• New `tests/syslog_test.py::TestSyslog::test_enable_rate_limit_feature_empty_feature_list` asserts the new "nothing to enable" message and that no shell commands run when the resolved list is empty.
• Existing `test_enable_syslog_rate_limit_feature`, `test_disable_syslog_rate_limit_feature`, and `tests/syslog_multi_asic_test.py` paths are unaffected.

On a single-ASIC DUT:

• `sudo config syslog rate-limit-feature enable syncd` prints `Enabling syslog rate limit feature for syncd` and `Enabled syslog rate limit feature for syncd`.
• `docker exec -i syncd supervisorctl status containercfgd` → `RUNNING`.
• `sudo config syslog rate-limit-container syncd -i 10 -b 500` → `docker exec -i syncd grep SystemLogRateLimit /etc/rsyslog.conf` shows `10` / `500`, and `grep containercfgd /var/log/syslog` shows the `Configure syslog rate limit interval=10, burst=500` notice.
• `sudo config syslog rate-limit-feature disable syncd` cleanly tears everything down.

On a multi-ASIC DUT: existing behavior is unchanged (`syncd0`, `syncd1`, … resolve as before; `bgp` global-only still resolves to `bgp`; `teamd` with both flags still resolves to `teamd` + per-ASIC names).

Previous command output (if the output of a command-line utility has changed)

On a single-ASIC DUT, before the fix:

```
admin@sonic:$ sudo config syslog rate-limit-feature enable syncd
admin@sonic:$
```

(Silent no-op — nothing printed, container untouched.)

New command output (if the output of a command-line utility has changed)

```
admin@sonic:~$ sudo config syslog rate-limit-feature enable syncd
Enabling syslog rate limit feature for syncd
Enabled syslog rate limit feature for syncd
```

And for a feature with neither scope flag set:

```
admin@sonic:~$ sudo config syslog rate-limit-feature enable ghost
No container resolved for service 'ghost' in namespace None, nothing to enable
```

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: nsoma-cisco / name: Nageswara Soma (78d9772)

[mssonicbld]

/azp run

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