feat: fix opencode parsing

[35C4n0r]

No description provided.

[github-actions]

✅ Preview binaries are ready!

To test with modules: agentapi_version = "agentapi_137" or download from: https://github.com/coder/agentapi/releases/tag/agentapi_137

[johnstcn]

Is this still a 'hack' as per the PR title? Has your testing turned up any further information?

[Copilot]

Pull Request Overview

This PR fixes the parsing logic for opencode message boxes to correctly identify and remove the input UI elements from terminal snapshots.

• Updated the removeOpencodeMessageBox function to detect the message box boundary more accurately
• Changed detection logic from checking for an empty line before ┃┃ to finding the first ┃┃ pattern that doesn't have another ┃┃ two lines above it
• Added test data to verify the fix handles partial renders correctly

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
lib/msgfmt/message_box.go	Fixed opencode message box detection logic to properly identify the start of the input box UI by checking for non-┃┃ content two lines above instead of just an empty line
lib/msgfmt/testdata/format/opencode/partial_render/msg.txt	Added test input data showing a partial render of opencode UI with message box
lib/msgfmt/testdata/format/opencode/partial_render/expected.txt	Added expected output after message box removal, showing only the content before the input UI
lib/httpapi/server.go	Added blank line for improved code readability

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