Gate on full config_db table coverage

[ideaship]

Part of the series tracked in #2562, which explains the ordering and what each PR covers. Based on the preceding PR in the stack, so review only the top commits here.

Thirty lines, and the reason the coverage number cannot quietly rot: a unit test
asserting that no table the generator can emit is left without a golden.

It closes the one coverage failure nothing else catches — a newly emitted
table arriving with no golden. Coverage lost the other way (a table that had a
golden becoming empty) already fails the golden comparison, because the golden
file itself changes.

It lives in the unit suite rather than the E2E job because it needs neither
NetBox nor a generated config, only the generator source and the committed
goldens. So it costs milliseconds and runs on every change, where the E2E job
runs on a file matcher and a 2400s budget. No .zuul.yaml change is needed.

It lands last because it can only pass once the golden set is complete — which
is what the PR below achieves.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="tests/unit/e2e/test_golden_coverage.py" line_range="27-29" />
<code_context>
+        "the generator can emit these config_db tables, but they are empty in "
+        "every file under tests/e2e/golden/: " + ", ".join(missing) + ". Seed a "
+        "device that populates them under tests/e2e/scenario/resources/ and "
+        "rewrite the goldens with `make sonic-e2e-regen`, or -- if the table "
+        "cannot be reached by any fixture -- exclude it here with a comment "
+        "saying why."
+    )
</code_context>
<issue_to_address>
**issue (bug_risk):** The failure message instructs maintainers to exclude an unreachable table, but the test has no exclusion set or mechanism; adding a comment alone does not change `missing`, so the test remains permanently failing for any genuinely unreachable emitted table.

**Triggers:** When a table is detected by the static generator scan but cannot be populated by any available fixture.

**Suggested fix:** Add an explicit, reviewed exclusion set containing the table and enforce that every exclusion has a nearby reason, or remove the unsupported exclusion advice from the message.

```suggestion
        "rewrite the goldens with `make sonic-e2e-regen`."
```
</issue_to_address>

Sourcery assessment

Approval pending. 1 finding to address first.

Blocking findings: tests/unit/e2e/test_golden_coverage.py:29

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[sourcery-ai]

issue (bug_risk): The failure message instructs maintainers to exclude an unreachable table, but the test has no exclusion set or mechanism; adding a comment alone does not change missing, so the test remains permanently failing for any genuinely unreachable emitted table.

Triggers: When a table is detected by the static generator scan but cannot be populated by any available fixture.

Suggested fix: Add an explicit, reviewed exclusion set containing the table and enforce that every exclusion has a nearby reason, or remove the unsupported exclusion advice from the message.

Suggested change

	"rewrite the goldens with `make sonic-e2e-regen`, or -- if the table "
	"cannot be reached by any fixture -- exclude it here with a comment "
	"saying why."
	"rewrite the goldens with `make sonic-e2e-regen`."

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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