chore: rename user-facing AI Bridge strings to AI Gateway (#26700)

Rename user-facing "AI Bridge" strings to "AI Gateway" in deployment
config, RBAC display names, log messages, error strings, docs style
guide, and Grafana dashboard README.

Deprecated option names and descriptions (the `--aibridge-*` block) are
intentionally kept as "AI Bridge". The `Name` field cannot be renamed
because `serpent` uses it as a unique key during JSON serialization;
duplicating names causes `UnmarshalJSON` failures (e.g. in the support
bundle). Descriptions also stay as "AI Bridge" to avoid confusion
between the deprecated and primary options.

Refs https://linear.app/codercom/issue/AIGOV-226

> Generated with the assistance of Coder Agents (@ssncferreira)

cli/testdata/coder_server_--help.golden

@@ -117,7 +117,7 @@ AI GATEWAY OPTIONS:
           limit, and is currently the only supported value.
 
       --ai-gateway-dump-dir string, $CODER_AI_GATEWAY_DUMP_DIR
-          Base directory for dumping AI Bridge request/response pairs to disk
+          Base directory for dumping AI Gateway request/response pairs to disk
           for debugging. When set, each provider writes under a subdirectory
           named after the provider. Sensitive headers are redacted. Leave empty
           to disable.

cli/testdata/server-config.yaml.golden

@@ -972,7 +972,7 @@ ai_gateway:
   # X-Ai-Bridge-Actor-Metadata-Username (their username).
   # (default: false, type: bool)
   send_actor_headers: false
-  # Base directory for dumping AI Bridge request/response pairs to disk for
+  # Base directory for dumping AI Gateway request/response pairs to disk for
   # debugging. When set, each provider writes under a subdirectory named after the
   # provider. Sensitive headers are redacted. Leave empty to disable.
   # (default: <unset>, type: string)