docs(client): clarify private_key_jwt custom claim precedence

[MukundaKatta]

Summary

• clarify that PrivateKeyJwtProviderOptions.claims adds custom JWT claims but does not override SDK-controlled standard claims
• add regression coverage for overlapping iss, sub, aud, exp, iat, and jti custom claims

Refs #1914.

Validation

• git diff --check
• pnpm --filter @modelcontextprotocol/client test -- test/client/authExtensions.test.ts
• pre-push hook: pnpm -r build, pnpm -r typecheck, pnpm sync:snippets --check && pnpm -r lint

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Latest commit: 2430289

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commit: 2430289