DX-120297: updating the MCP to verify ES256 signatures

[ajulka-drem]

Summary

• Supporting ES256 signed tokens (ECDSA); the MCP JWKS verifier previously only accepted RS256
• Updated algorithms list in JWKSVerifier.verify() to accept RS256, ES256, ES384, and ES512, covering both legacy RSA and all ECDSA variants.
• Added tests/servers/test_jwks_integration.py — a skipped-by-default integration test harness that validates a real JWKS endpoint and JWT, including
  tampered-token and garbage-token cases.

Test plan

• Fill in JWKS_URL and TOKEN in tests/servers/test_jwks_integration.py with a live ES256-signed token and run uv run pytest
  tests/servers/test_jwks_integration.py -v -s to confirm the verifier accepts the token and returns valid claims.
• Confirm a tampered token returns None (test already covers this).
• Confirm existing unit tests still pass: uv run pytest tests/.

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[ssaumitra]

lgtm