Accept declared Runtime observation reasons

[SaladDay]

A Provider can declare Observe unsupported with a valid authored reason such as native_metrics_not_supported. Core already preserves that result, but the management schema and client rejected it, and Web assumed every reason had a translation.

Accept the Provider contract’s safe reason codes in both observation schemas and the client’s managed-unsupported branch. Preserve existing identity, null-sample and status/reason rules; unknown safe codes display directly in Web while known reasons retain their translations. Remove the obsolete generated reason enums and update the owning English/Chinese protocol documentation.

Validation: shared Go/service/HTTP/schema/client fixtures cover valid and malformed codes, length bounds and trailing newlines; actual Go classifications are checked against the client’s existing unavailable-reason set. Independent blind review and coordinator full-diff review passed. The only schema changes are the two management observation reason properties; the pinned public contract is unchanged.

Checks: module build/vet; 2,391 passing Go tests/subtests with three explicit platform/helper gates; 766 client tests; 381 Web tests and two English/Chinese component regressions; client/Web typechecks; OpenAPI generation/drift, documentation and naming checks; website build and 22 site tests.

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