feat(server): add mcpToolNameResolver helper for signature auth

[bokelley]

Summary

• Export mcpToolNameResolver from @adcp/client/server — a default resolveOperation callback for MCP agents wiring RFC 9421 signature auth. Parses req.rawBody as JSON-RPC and returns params.name when method === 'tools/call'.
• Rewrite the four JSDoc examples in src/lib/server/auth-signature.ts to use the helper instead of copy-pasting the same 6-line parser.
• Unit tests cover the happy path plus every undefined-returning case (non-tools/call method, missing params, non-string name, malformed JSON, missing/empty rawBody).

Motivation

The RFC 9421 signing composition helpers (verifySignatureAsAuthenticator, requireSignatureWhenPresent, requireAuthenticatedOrSigned) take a resolveOperation callback whose MCP implementation is identical across every seller agent:

resolveOperation: req => {
  try {
    const body = JSON.parse(req.rawBody ?? '');
    if (body.method === 'tools/call') return body.params?.name;
  } catch {}
  return undefined;
}

That block appears four times in auth-signature.ts JSDoc alone, plus once verbatim in test/auth-signature-compose.test.js, plus every time it's pasted into a new adapter. Ships as a one-line import instead:

resolveOperation: mcpToolNameResolver,

A2A agents use a different envelope and still need a bespoke resolver — the helper is explicitly MCP.

Follow-up to #914 (docs-only signing guide): once this lands I'll propose a rewrite of Step 4 of SIGNING-GUIDE.md using requireAuthenticatedOrSigned + mcpToolNameResolver, collapsing the ~25-line manual composition to ~8 lines.

Test plan

• New tests: node --test test/lib/mcp-tool-name-resolver.test.js (7/7 pass locally)
• Regression: existing auth-signature-compose.test.js still green (37/37)
• npm run build clean
• tsc --noEmit clean (ran in pre-push)

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