feat: add Google Chat channel with Pub/Sub pull, Cards V2, Drive upload

[duhd-vnpay]

Summary

Full Google Chat channel integration using pure HTTP (no Google client libraries):

• Auth: Service Account JWT → access token with auto-refresh (60s pre-expiry)
• Inbound: Pub/Sub REST pull loop with dedup cache, base64 event parsing, DM/group routing
• Outbound: Markdown→Google Chat format conversion (bold, italic, strikethrough, links), byte-aware chunking (3900B safety limit for Vietnamese/CJK), Cards V2 for tables
• Media: Chat API attachment download, Google Drive multipart upload for long-form responses, retention cleanup loop
• Send pipeline: Placeholder edit pattern (Thinking... → final), exponential backoff retry on 429/5xx, thread routing for groups
• Factory: ChannelFactory pattern for DB-instance hot-reload, config-file fallback registration
• Phase 2 stubs: webhook.go (push mode), stream.go (progressive edit)

New Files (14)

File	Purpose
internal/channels/googlechat/constants.go	API URLs, limits, scopes, retry config
internal/channels/googlechat/auth.go	Service Account JWT auth with auto-refresh
internal/channels/googlechat/auth_test.go	6 auth tests
internal/channels/googlechat/format.go	Markdown conversion, byte-aware chunking, summary extraction
internal/channels/googlechat/format_test.go	19 format tests (markdown, chunking, Unicode, summary)
internal/channels/googlechat/pubsub.go	Pub/Sub pull loop, event parsing, dedup cache
internal/channels/googlechat/pubsub_test.go	9 pubsub tests
internal/channels/googlechat/media.go	Attachment download, Drive upload, retention cleanup
internal/channels/googlechat/send.go	Send pipeline, Cards V2, placeholder edit, retry
internal/channels/googlechat/send_test.go	3 send tests
internal/channels/googlechat/googlechat.go	Channel struct, constructor, Start/Stop lifecycle
internal/channels/googlechat/factory.go	ChannelFactory for DB-instance hot-reload
internal/channels/googlechat/webhook.go	Phase 2 stub
internal/channels/googlechat/stream.go	Phase 2 stub

Modified Files (4)

File	Change
internal/channels/channel.go	Added TypeGoogleChat = "google_chat"
internal/config/config_channels.go	Added GoogleChatConfig struct (18 fields)
cmd/gateway_channels_setup.go	Config-based Google Chat registration
cmd/gateway.go	Factory registration for DB instances

Test Results (28/28 PASS)

=== RUN   TestNewServiceAccountAuth_ValidFile          --- PASS (0.09s)
=== RUN   TestNewServiceAccountAuth_InvalidFile        --- PASS (0.02s)
=== RUN   TestNewServiceAccountAuth_MissingFile         --- PASS (0.00s)
=== RUN   TestServiceAccountAuth_Token_CachesWithinTTL --- PASS (0.11s)
=== RUN   TestServiceAccountAuth_Token_RefreshesExpired --- PASS (0.28s)
=== RUN   TestServiceAccountAuth_Token_RefreshFailure  --- PASS (0.06s)
=== RUN   TestMarkdownToGoogleChat (10 sub-tests)      --- PASS (0.00s)
=== RUN   TestDetectStructuredContent (5 sub-tests)    --- PASS (0.00s)
=== RUN   TestChunkByBytes (3 sub-tests)               --- PASS (0.00s)
=== RUN   TestChunkByBytes_Unicode                     --- PASS (0.00s)
=== RUN   TestExtractSummary (5 sub-tests)             --- PASS (0.00s)
=== RUN   TestParseEvent_Message                       --- PASS (0.00s)
=== RUN   TestParseEvent_GroupSpace                    --- PASS (0.00s)
=== RUN   TestParseEvent_AddedToSpace                  --- PASS (0.00s)
=== RUN   TestParseEvent_MalformedJSON                 --- PASS (0.00s)
=== RUN   TestParseEvent_EmptyData                     --- PASS (0.00s)
=== RUN   TestParseEvent_MissingSender                  --- PASS (0.00s)
=== RUN   TestParseEvent_BotSelfFilter                 --- PASS (0.00s)
=== RUN   TestParseEvent_WithAttachment                --- PASS (0.00s)
=== RUN   TestDedupCache                               --- PASS (0.00s)
=== RUN   TestSendMessage_ShortDM                      --- PASS (0.00s)
=== RUN   TestSendMessage_EmptyContent                 --- PASS (0.00s)
=== RUN   TestBuildCardMessage_Table                   --- PASS (0.00s)
PASS
ok  github.com/nextlevelbuilder/goclaw/internal/channels/googlechat  0.625s

Test plan

• Verify go build ./... compiles cleanly
• Verify go test ./internal/channels/googlechat/... passes (28 tests)
• Verify no regressions in existing channel tests
• Integration test with real Google Chat workspace (requires SA + Pub/Sub setup)

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