Add server conformance for SEP-2350 scope challenges

Goal

Add one language-neutral server conformance scenario for request-time OAuth insufficient_scope behavior from SEP-2350. The scenario should exercise every invocable MCP server primitive: tools/call, resources/read for both a static URI and a template-expanded URI, and prompts/get.

Portable fixture contract

Reuse the existing cross-SDK fixtures so implementations fail on observable wire behavior rather than on a missing test-only primitive:

Servers should treat mcp-conformance-scope-low as a valid opaque token with only mcp:conformance:baseline, and mcp-conformance-scope-full as a valid opaque token with all required scopes. Unauthenticated behavior remains unchanged for existing scenarios.

For each operation, the low token should produce HTTP 403 and a Bearer WWW-Authenticate challenge containing error="insufficient_scope", the fixture protected-resource metadata URL, and both required scopes in one challenge. Retrying the same operation with the full token should produce the normal successful MCP result.

Rationale

src/seps/sep-2350.yaml currently excludes the server single-challenge requirement because the challenge was the only source of truth for required scopes. The fixed fixture contract supplies independent ground truth, so sep-2350-server-single-challenge can become an observable check.

This deliberately does not test general token validation or require a mock authorization server; it isolates the SDK/server request-time challenge seam. It also avoids the TypeScript-specific client/auth imports and guessed admin-action fixture in the older, unmerged #106 approach.

Proof

Run the official scenario through the existing SDK runner against:

1. Unmodified modelcontextprotocol/typescript-sdk@main: build succeeds, but low-token operations return HTTP 200 and the scenario exits non-zero.
2. SamMorrowDrums/typescript-sdk@scope-challenge-server-sdk (modelcontextprotocol/typescript-sdk#1624): all challenge and upgraded-retry checks pass.

Related

• SEP-2350: modelcontextprotocol/modelcontextprotocol#2350
• Existing client coverage: #281 / #282
• Existing server-row exclusion: #302
• Older overlapping proposal: #106
• TypeScript SDK implementation: modelcontextprotocol/typescript-sdk#1624

Start with src/seps/sep-2350.yaml, the existing cross-SDK fixtures, and the official scenario runner. Exercise tools/call, static and template resources/read, and prompts/get with the low and full tokens. Done means low-token requests return HTTP 403 with one insufficient_scope challenge containing both required scopes, while full-token retries succeed across the listed operations.