chore: deprecate injected MCP approach in AI Bridge (#23031)

_Disclaimer: implemented by a Coder Agent using Claude Opus 4.6._

Marks the injected MCP approach in AI Bridge as deprecated across the
codebase.

## Changes

- **`codersdk/deployment.go`**: Deprecated `ExternalAuthConfig.MCPURL`,
`.MCPToolAllowRegex`, `.MCPToolDenyRegex` fields; deprecated and hid the
`--aibridge-inject-coder-mcp-tools` server flag; deprecated
`AIBridgeConfig.InjectCoderMCPTools`.
- **`coderd/externalauth/externalauth.go`**: Deprecated `Config.MCPURL`,
`.MCPToolAllowRegex`, `.MCPToolDenyRegex`.
- **`enterprise/aibridgedserver/aibridgedserver.go`**: Added runtime
deprecation warning when `CODER_AIBRIDGE_INJECT_CODER_MCP_TOOLS` is
enabled; deprecated `getCoderMCPServerConfig`.
- **`enterprise/aibridged/mcp.go`**: Deprecated `MCPProxyBuilder`
interface and `MCPProxyFactory` struct.
- **`docs/ai-coder/ai-bridge/mcp.md`**: Added deprecation warning
banner.

cli/testdata/coder_server_--help.golden

@@ -143,11 +143,6 @@ AI BRIDGE OPTIONS:
       --aibridge-enabled bool, $CODER_AIBRIDGE_ENABLED (default: false)
           Whether to start an in-memory aibridged instance.
 
-      --aibridge-inject-coder-mcp-tools bool, $CODER_AIBRIDGE_INJECT_CODER_MCP_TOOLS (default: false)
-          Whether to inject Coder's MCP tools into intercepted AI Bridge
-          requests (requires the "oauth2" and "mcp-server-http" experiments to
-          be enabled).
-
       --aibridge-max-concurrency int, $CODER_AIBRIDGE_MAX_CONCURRENCY (default: 0)
           Maximum number of concurrent AI Bridge requests per replica. Set to 0
           to disable (unlimited).