Add Spector coverage for date-like extensible enum names

[Copilot]

The existing type/enum/extensible coverage only used word-like enum member names, missing regressions where date/version-like names can be coerced when round-tripped through YAML.

• Extensible enum scenario
  • Added ApiVersionExtensibleEnum with date-like member names and string values:

union ApiVersionExtensibleEnum {
  string,
  `2020-01-01`: "2020-01-01",
  `2021-01-01`: "2021-01-01",
}

• Spector operations

  • Added GET/PUT coverage for known and unknown API version values under /type/enum/extensible/api-version.

• Mock API coverage

  • Added matching mock responses and request expectations for "2020-01-01" and "2022-01-01".

[msyyc]

@copilot make sure you pass CI

[Copilot]

Validated commit 94d19fe: PR check runs show typespec - ci completed successfully. Local focused checks also pass: pnpm -r --filter "@typespec/http-specs..." build, pnpm --filter @typespec/http-specs validate-mock-apis, and Prettier for the changed files.

[timotheeguerin]

I think @iscai-msft was working on a generic special char spec somewehere, I don't think this test belong in this spec, same as we have a SpecialWords one we should have a SpecialChart one which covers those things.

Also api version doesn't make sense here from a core typespec http spec, this is just about having numbers and special chars