fix(im/wecom): preserve image in mixed messages with text captions

[loganv]

Description

WeCom long-connection adapter dropped the image whenever a mixed (text+image) message contained any text. The most common mixed-message shape — a question plus a screenshot — reached the QA pipeline as text-only, so the model never saw the image, despite the IM integration docs stating images are handled as QA attachments.

convertMixedMessage now keeps the first image as the message attachment and carries the text as its caption (MessageTypeImage + Content). The IM service already supports captioned images: prepareIMAttachments downloads the file while fileMessageQAContent preserves msg.Content as the query, so no changes outside the wecom adapter are needed.

Text-only, image-only, and empty mixed messages keep their previous behavior. Multiple images still collapse to the first one — supporting several attachments requires a multi-attachment IncomingMessage and is left out of this minimal fix.

Type of Change

• 🐛 Bug fix

Related Issue

Fixes #

Testing

• New table-driven tests in internal/im/wecom/mixed_test.go covering: text+image, multiple images with text, text-only, image-only, group-chat @mention stripping, and empty mixed message
• go test ./internal/im/wecom/ — pass
• go test ./internal/im/... (all platform adapters: feishu, slack, telegram, mattermost, qqbot, yunzhijia, wecom) — pass, no regression
• go vet ./internal/im/wecom/ — clean
• gofmt -l internal/im/wecom/ — clean
• git diff --check origin/main...HEAD — clean

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable (for Go: golangci-lint run --new-from-rev=origin/main ./...)
• Full-repository checks were run, or any unrelated/environment-dependent failures are documented above
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (README, website-docs/, Swagger annotations, etc.)
• Breaking changes are clearly called out in the description above

Note: golangci-lint is not installed locally on this machine (network-restricted environment); the change is gofmt/go vet clean and covered by unit tests. The behavior now matches what website-docs/03-features/12-im-integration.md already documents (fileMessageQAContent caption path), so no doc update is needed.