starknet_transaction_prover: add TLS integration tests

Add 5 tests for the TLS server module:
- HTTPS request succeeds with self-signed cert (specVersion)
- Plain HTTP to TLS server fails (protocol mismatch)
- Missing cert/key file produces error
- Invalid PEM content produces error

Uses rcgen (new workspace dev-dep) to generate self-signed certs at
test time, avoiding committed certificate fixtures that expire.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: avi-starkware