test: cover legacy extension capability negotiation

[sambhav]

capabilities.extensions can be lost during SDK serialization or parsing even when the legacy initialize handshake succeeds. This adds coverage for the behavior clarified in modelcontextprotocol/modelcontextprotocol#3364 and defined by SEP-2133.

Closes #517.

Changes

• Add one client scenario (legacy-extensions) and one server scenario (server-legacy-extensions), each exercising the 2025-11-25 handshake and capability preservation in both directions.
• Check the Apps capability, empty settings objects, and arbitrary nested test settings, including arrays, false, zero, and null. Client and server fixtures use distinct settings.
• Require a diagnostic tool report from the SDK's capability accessor so deserialization loss is observable. Missing reports fail as untestable instead of silently skipping.
• Inspect server initialization responses directly on the wire and reject a different negotiated version. Support JSON and SSE responses and clean up sessions.
• Extend the TypeScript everything-client/server and add negative controls for dropped advertisements, dropped received capabilities, stripped settings, missing reports, and a different negotiated version.
• Register the scenarios in optional extension suites, outside core/Tier-1 and dated-spec selections. Document the fixture contract and commands in SDK_INTEGRATION.md.

This tests capability preservation on 2025-11-25, not full Apps functionality, all earlier protocol versions, or the newer per-request capability flow. The example.com identifiers are diagnostic fixtures only.

Validation

• npm run build
• npm run check (typecheck and lint)
• npm test: 49 files, 636 tests passed
• Client CLI run against the TypeScript everything-client: 8/8 checks passed
• Server scenario against the TypeScript everything-server: passed; included in the integration suite
• git diff --check

The environment blocks the tsx CLI's IPC socket, so local test execution used a node_modules-only launcher adapter invoking node --import tsx. No dependency, lockfile, or repository script changes are included.

AI assistance: Codex prepared and validated this change at Sambhav's request.

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commit: 2c19a18