fix(tui): include attachment path in model context - #41455

Issue for this PR

Closes #41454

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Preserves a local attachment's source.path as a text part immediately before the binary image in model context. Providers that discard image filenames can therefore still expose the user-selected path to the agent. A focused regression test covers the V2 message conversion. Prepared with AI assistance and validated locally.

How did you verify your code works?

• bun test test/session/message-v2.test.ts --timeout 30000 — 40 pass
• bun test test/session/llm.test.ts --timeout 30000 — 28 pass
• bun typecheck
• bun run prettier --ignore-unknown --check packages/opencode/src/session/message-v2.ts packages/opencode/test/session/message-v2.test.ts
• bun run oxlint packages/opencode/src/session/message-v2.ts packages/opencode/test/session/message-v2.test.ts — 0 errors; pre-existing warnings only
• git diff --check

The full dependency install reached the unrelated tree-sitter-powershell postinstall, which requires a Visual Studio C++ workload unavailable in this environment; the targeted tests and package typecheck above completed successfully.

Screenshots / recordings

Not applicable; this changes model context only and has no visual UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR