[Bug]: Dockerfile paths lose syntax highlighting in tool results and diffs

Start at getLanguageFromPath() and the existing resolver regression tests; compare bare, relative, POSIX, and Windows paths, including dotted directories and Dockerfile.bak. Use the .pr/dockerfile-language/fixture.patch and README with npm run dev:mock if validating the UI. Done means the regression tests pass and file-editor results and diffs restore Dockerfile highlighting.

Operating System

Windows

Installation Method

From source (npm run dev / npm run dev:minimal)

Agent Canvas Version

1.16.0 / main at fe09f319b0e66dbbcd2779e6b44c928d8516b44d

Bug Description

getLanguageFromPath() recognizes a bare Dockerfile, but returns text for deploy/Dockerfile, an absolute POSIX path, or a Windows path. File-editor tool results consequently show Dockerfile contents without syntax highlighting. The Monaco diff viewer uses the same resolver. The function splits the entire path on dots without first extracting its final filename.

Steps to Reproduce

Reproduced in the actual Canvas UI with npm run dev:mock on Windows, Node 24.19.0. No live LLM is needed.

1. Check out main at fe09f319b0e66dbbcd2779e6b44c928d8516b44d and run npm ci.
2. Apply the reproduction fixture patch. It changes the existing canvas-demo fixture to a file-editor observation for /workspace/project/deploy/Dockerfile.
3. Run npm run dev:mock, open http://localhost:3001, add the suggested local mock backend, and skip LLM setup.
4. Open Generated canvas demo and expand Wrote to Dockerfile/.

The fixture supplies data only; the real chat and syntax-highlighter components render the result. Full reproduction notes.

Actual Behavior

The Dockerfile instructions and strings are all rendered as plain text.

The LLM setup banner is expected in mock mode; this reproduction concerns rendering a persisted tool result.

Expected Behavior

A Dockerfile should select the same language whether its path is a bare filename, relative, or absolute, with either POSIX or Windows separators. Its instructions and strings should retain syntax highlighting.

Relevant Logs

getLanguageFromPath("Dockerfile") => "dockerfile"
getLanguageFromPath("deploy/Dockerfile") => "text"
getLanguageFromPath("/workspace/project/Dockerfile") => "text"
getLanguageFromPath("C:\workspace\project\Dockerfile") => "text"

Regression tests against the base resolver: 4 failed, 8 passed.

Acceptance Criteria

• Relative and absolute Dockerfile paths resolve to dockerfile with both POSIX and Windows separators.
• Directory names containing dots do not affect basename detection.
• Existing extension detection, unknown files, and backup files such as Dockerfile.bak retain their behavior.
• Regression tests fail on the base resolver and pass with the fix.
• Before-and-after screenshots demonstrate restored highlighting in a real file-editor tool result.

Screenshots

No response

Additional Context

No response