feat(plugins/google_chat): Google Chat platform adapter as a bundled plugin

Adds Google Chat as a new gateway platform, shipped under
plugins/platforms/google_chat/ following the canonical bundled-plugin
pattern (Teams, IRC).  Rewired from the original PR #18425 to use the
new env_enablement_fn + cron_deliver_env_var plugin interfaces landed
in the preceding commit, so the adapter touches ZERO core files.

What it does:
- Inbound DM + group messages via Cloud Pub/Sub pull subscription (no
  public URL needed), with attachments (PDFs, images, audio, video)
  downloaded through an SSRF-guarded Google-host allowlist.
- Outbound text replies with the 'Hermes is thinking…' patch-in-place
  pattern — no tombstones.
- Native file attachment delivery via per-user OAuth.  Google Chat's
  media.upload endpoint rejects service-account auth, so each user
  runs /setup-files once in their own DM to grant
  chat.messages.create for themselves; the adapter then uploads as
  them.  Tokens stored per email at
  ~/.hermes/google_chat_user_tokens/<email>.json.
- Thread isolation: side-threads get isolated sessions, top-level DM
  messages share one continuous session.  Persistent thread-count
  store survives gateway restart.
- Supervisor reconnect with exponential backoff.
- Multi-user out of the box.

How it plugs in (no core edits):
- env_enablement_fn seeds PlatformConfig.extra with project_id,
  subscription_name, service_account_json, and the home_channel dict
  (which the core hook turns into a HomeChannel dataclass).  Reads
  GOOGLE_CHAT_PROJECT_ID (falls back to GOOGLE_CLOUD_PROJECT),
  GOOGLE_CHAT_SUBSCRIPTION_NAME (falls back to GOOGLE_CHAT_SUBSCRIPTION),
  GOOGLE_CHAT_SERVICE_ACCOUNT_JSON (falls back to
  GOOGLE_APPLICATION_CREDENTIALS), GOOGLE_CHAT_HOME_CHANNEL.
- cron_deliver_env_var='GOOGLE_CHAT_HOME_CHANNEL' gets cron delivery
  for free — cron/scheduler.py consults the platform registry for any
  name not in its hardcoded built-in sets.
- plugin.yaml's rich requires_env / optional_env blocks auto-populate
  OPTIONAL_ENV_VARS via the new hermes_cli/config.py injector, so
  'hermes config' UI surfaces them with description / url / prompt /
  password metadata.
- Module-level Platform('google_chat') call in adapter.py triggers the
  Platform._missing_() registration so Platform.GOOGLE_CHAT attribute
  access works without an enum entry.

Distribution: ships inside the existing hermes-agent package.  Users
opt in via 'pip install hermes-agent[google_chat]' and follow the
8-step GCP walkthrough at
website/docs/user-guide/messaging/google_chat.md.

Test coverage: 153 tests in tests/gateway/test_google_chat.py, all
passing.  Spans platform registration, env config loading, Pub/Sub
envelope routing, outbound send + chunking + typing patch-in-place,
attachment send paths, SSRF guard, thread/session model,
supervisor reconnect, authorization, per-user OAuth, and the new
plugin-registry cron delivery wiring.

Credit: adapter + OAuth + tests + docs authored by @donramon77
(PR #18425).  Rewire onto the new plugin hooks + salvage commit by
Teknium.

Co-Authored-By: Ramón Fernández <112875006+donramon77@users.noreply.github.com>

.env.example

@@ -423,3 +423,24 @@ IMAGE_TOOLS_DEBUG=false
 # TEAMS_HOME_CHANNEL=                  # Default channel/chat ID for cron delivery
 # TEAMS_HOME_CHANNEL_NAME=             # Display name for the home channel
 # TEAMS_PORT=3978                      # Webhook listen port (Bot Framework default)
+
+# =============================================================================
+# GOOGLE CHAT INTEGRATION
+# =============================================================================
+# Connects via Cloud Pub/Sub pull subscription (no public URL required).
+# Setup walkthrough: website/docs/user-guide/messaging/google_chat.md.
+# 1. Create a GCP project, enable the Google Chat API and Cloud Pub/Sub.
+# 2. Create a Service Account with roles/pubsub.subscriber on the
+#    subscription (NOT project-wide); download the JSON key.
+# 3. Configure your Chat app at console.cloud.google.com/apis/credentials
+#    → Google Chat API → Configuration → Cloud Pub/Sub topic.
+# 4. (Optional, for native attachment delivery) Each user runs
+#    `/setup-files` once in their own DM after Pub/Sub is wired up.
+#
+# GOOGLE_CHAT_PROJECT_ID=                       # GCP project hosting the topic (or set GOOGLE_CLOUD_PROJECT)
+# GOOGLE_CHAT_SUBSCRIPTION_NAME=                # Full path: projects/<id>/subscriptions/<name>
+# GOOGLE_CHAT_SERVICE_ACCOUNT_JSON=             # Path to SA JSON (or set GOOGLE_APPLICATION_CREDENTIALS)
+# GOOGLE_CHAT_ALLOWED_USERS=                    # Comma-separated emails allowed to talk to the bot
+# GOOGLE_CHAT_ALLOW_ALL_USERS=false             # Set true to skip the allowlist
+# GOOGLE_CHAT_HOME_CHANNEL=                     # Default space (spaces/XXXX) for cron delivery
+# GOOGLE_CHAT_HOME_CHANNEL_NAME=                # Display name for the home channel