feat: ✨ bridge LINE Flex message fallbacks

[clins1994]

Summary

• Treat LINE content type 22 as bridgeable
• Bridge Flex/bot messages as Matrix notices that tell users to check LINE for full details
• Include ALT_TEXT as a preview when LINE provides it

Fixes #126

Testing

• go test ./pkg/connector
• git diff --check

[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added support for LINE Flex messages in the Matrix bridge. Flex messages are now properly recognized and converted to a readable text format, including alternative text previews when available.

Walkthrough

This PR adds support for bridging LINE Flex/rich messages (content_type=22) to Matrix. It defines a new ContentFlex constant, updates message validation to accept the type, routes Flex messages to a new conversion handler, and implements fallback conversion using ALT_TEXT metadata or a default notice.

Changes

Flex Message Bridging

Layer / File(s)	Summary
Content type constant definition pkg/connector/consts.go	ContentFlex constant added to ContentType enum with value 22.
Message routing and handler integration pkg/connector/handle_message.go	isBridgeableContentType now includes ContentFlex in supported types; convertLineMessage dispatch adds case to route ContentFlex to h.ConvertFlex(...).
Flex message conversion handler pkg/connector/handlers/flex.go	ConvertFlex method converts LINE Flex messages to Matrix fallback ConvertedMessage using trimmed ALT_TEXT from metadata or default "LINE rich message" notice.

Sequence Diagram

sequenceDiagram
  participant LineMessage as LINE Message
  participant MessageHandler as convertLineMessage
  participant BridgeCheck as isBridgeableContentType
  participant FlexConverter as ConvertFlex
  participant MatrixMessage as Matrix Message

  LineMessage->>MessageHandler: receive with ContentFlex
  MessageHandler->>BridgeCheck: check if ContentFlex is supported
  BridgeCheck-->>MessageHandler: returns true
  MessageHandler->>FlexConverter: route ContentFlex
  FlexConverter-->>MatrixMessage: build fallback with ALT_TEXT or notice

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related issues

• #126: feat: ✨ Bridge LINE Flex/bot messages — This PR directly implements the feature request to bridge LINE content_type=22 Flex messages by adding ContentFlex constant, routing logic, and fallback conversion using ALT_TEXT metadata, which matches the primary objective to stop dropping these messages and provide a text fallback.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: bridging LINE Flex messages as fallbacks to Matrix.
Description check	✅ Passed	The description is directly related to the changeset, outlining the three key changes and testing steps.
Linked Issues check	✅ Passed	The PR fully addresses issue #126 by making content_type=22 bridgeable and converting Flex messages to Matrix notices with ALT_TEXT preview when available.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to bridging LINE Flex messages: adding a constant, marking it bridgeable, and implementing conversion logic with ALT_TEXT fallback.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[clins1994]

Currently this is what happens

The fix will return a best effort preview and a check line for more details