fix: did:cid universal resolver responses

[macterra]

Summary

Fixes #699.

This tightens the Universal Resolver-style /1.0/identifiers/{did} response for did:cid in both Gatekeeper implementations.

• Rust Gatekeeper: adds DID media type negotiation, stable didResolutionMetadata.contentType, and fixture-stable metadata on successful conformant resolution responses
• TypeScript Gatekeeper: mirrors the Rust behavior in identifiers-router, including exact Content-Type responses for DID JSON and DID JSON-LD
• removes volatile didResolutionMetadata.retrieved from the conformant Universal Resolver-style success response while leaving the internal resolver metadata behavior intact
• keeps conformant responses limited to the DID Resolution triple: didDocument, didResolutionMetadata, and didDocumentMetadata
• updates Gatekeeper docs and records the Rust formatting lesson in AGENTS.md

Validation

• cargo test --manifest-path rust/services/gatekeeper/Cargo.toml --test http_contract -- --nocapture
• node --experimental-vm-modules node_modules/.bin/jest tests/gatekeeper/api.test.ts --runInBand --verbose

Note: the Rust test run still reports the existing unused import warning for queue_outbound_operation in rust/services/gatekeeper/src/lib.rs.

[Copilot]

Pull request overview

This PR tightens Gatekeeper’s Universal Resolver-style GET /1.0/identifiers/{did} responses for did:cid so they are fixture-stable and properly advertise DID media types across both the TypeScript and Rust Gatekeeper surfaces.

Changes:

• Adds stable didResolutionMetadata.contentType and strips volatile didResolutionMetadata.retrieved from successful resolution results.
• Honors Accept: application/did+json vs Accept: application/did+ld+json, returning the selected DID media type as the HTTP Content-Type.
• Extends both TS (Jest) and Rust (http_contract) tests plus documentation to lock in fixture-ready behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/gatekeeper/api.test.ts	Adds Jest coverage for contentType and omission of retrieved, including Accept negotiation assertions.
services/gatekeeper/server/src/identifiers-router.ts	Implements TS-side Accept negotiation, stable metadata shaping, and explicit DID media Content-Type on success.
rust/services/gatekeeper/tests/http_contract.rs	Adds Rust HTTP contract coverage for fixture-stable resolution results and negotiated DID media types.
rust/services/gatekeeper/src/api.rs	Implements Rust-side Accept negotiation, stable metadata shaping, and DID media Content-Type on success.
docs/services/gatekeeper/README.md	Documents the fixture-ready response behavior and negotiated DID media types.
AGENTS.md	Records a repo workflow lesson about avoiding broad cargo fmt churn in focused Rust Gatekeeper fixes.

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