Remove deprecated global clientTlsConfig from ServerRuntime

[Cali0707]

Depends on #286 #287

Description

Remove the deprecated runtime.clientTlsConfig field from ServerRuntime configuration.

After #286 (per-invocation TLS config) and #287 (deprecation warning) have been released and users have had time to migrate, remove the global config entirely.

Requirements

• Remove ClientTLSConfig field from ServerRuntime struct
• Remove GetHTTPClient() method from ServerRuntime
• Remove HTTP client middleware that injects global client
• Update HTTP invocation to only use per-invocation TLS config
• Remove related fields from JSON schema
• Update documentation to remove global config references
• Add clear error message if old config format is detected

Migration Error Message

ERROR: runtime.clientTlsConfig has been removed.
Please configure clientTlsConfig on individual HTTP invocations or invocationBases.
See migration guide: https://github.com/genmcp/gen-mcp/blob/main/docs/migration.md

Acceptance Criteria

• runtime.clientTlsConfig field removed from types
• Schema files updated (field removed)
• Clear error if user has old config
• Documentation updated
• All tests pass without global TLS config support