Deprecate global clientTlsConfig in ServerRuntime

[Cali0707]

Depends on #286

Description

Deprecate the global runtime.clientTlsConfig in favor of per-invocation TLS configuration on HttpInvocationConfig.

Once #286 is implemented, the global config becomes redundant and potentially confusing. Users should configure TLS at the invocation level for better security (principle of least privilege) and flexibility.

Requirements

• Add deprecation warning when runtime.clientTlsConfig is used
• Log warning at server startup indicating the field is deprecated
• Update documentation to recommend per-invocation config
• Update examples to use per-invocation pattern

Deprecation Message

DEPRECATED: runtime.clientTlsConfig is deprecated and will be removed in a future release.
Please configure clientTlsConfig on individual HTTP invocations or invocationBases instead.
See https://github.com/genmcp/gen-mcp/blob/main/docs/mcpserver.md#custom-ca-certificates

Acceptance Criteria

• Warning logged when global clientTlsConfig is configured
• Documentation updated with deprecation notice
• Migration guide added showing how to move from global to per-invocation config
• Examples updated to prefer per-invocation pattern