crypto/tls: add ECH provider rejection unit tests

[Abhicodeitout]

Summary

Add unit tests for ECH provider rejection behavior when the client-facing server receives malformed GREASE-like or unsupported algorithm combinations.

What changed

• Added a test case that uses a known config_id with an encapsulated key generated under a different KEM.
  • Expected behavior: provider returns reject (not abort).
• Added a test case that uses a known config_id with an unsupported KDF/AEAD pair.
  • Expected behavior: provider returns reject.
• Kept all changes in src/crypto/tls/ech_test.go.

Why

A reviewer suggestion on #95 asked for explicit coverage that the client-facing server rejects:

• GREASE ECH that matches config_id but uses a different KEM.
• Unsupported KDF/AEAD algorithm combinations.

These tests lock in the intended reject path for both cases.

Verification

Executed with repository toolchain:

GOROOT=/workspaces/go ../bin/go test crypto/tls -run TestECHProvider -count=1 -v

Result: PASS, including both new subtests.

Commit

Includes Signed-off-by: Abhishek Kumar Kushwaha abhithegabbar@gmail.com.

[Abhicodeitout]

@benburkert @evanphx @bmizerany can you review