fix(validation): accept absolute paths in validate_path_relative_to when working_dir is set

[clouatre]

fix(validation): accept absolute paths in validate_path_relative_to when working_dir is set

Problem

edit_overwrite rejected absolute paths even when the caller supplied an explicit working_dir. For example, writing AGENTS.md to /Users/hugues/git/career/ while the MCP server CWD is the aptu-coder repo failed with "path is outside the working directory".

Root cause

validate_path_relative_to delegated to validate_parent_in_root for the require_exists=false branch. That function enforces canonical_parent.starts_with(root) -- correct when called from validate_path to keep paths inside the server CWD, but wrong when an explicit working_dir is provided. The containment boundary for working_dir callers is the orchestrator's responsibility, not the server's; the server's job is path resolution. The delegated call rejected any parent that did not sit inside the server CWD, ruling out all absolute paths.

Fix

Replace the validate_parent_in_root delegation in validate_path_relative_to's require_exists=false branch with minimal path resolution:

1. Reject bare .. / . / trailing-slash paths (no file_name component).
2. Join the path's parent to canonical_working_dir (Path::join handles absolute right-hand sides by discarding the left side, so absolute paths resolve correctly).
3. canonicalize the parent -- resolves symlinks and requires the directory to exist.
4. Verify the canonicalized parent is a directory, not a file.
5. Return canonical_parent.join(file_name).

No containment check. validate_path_relative_to is a path resolver; access control is the orchestrator's job.

validate_parent_in_root is unchanged and continues to enforce CWD containment for validate_path (no working_dir).

Tests

• test_validate_path_in_dir_traversal_to_sibling_accepted_with_working_dir: asserts that ../sibling/file is accepted when the caller provides an explicit working_dir; containment enforcement is the orchestrator's responsibility.
• test_validate_path_in_dir_rejects_sibling_prefix_validate_path: confirms validate_path (CWD-scoped, no working_dir) still rejects paths outside the server CWD.
• test_validate_path_relative_to_accepts_absolute_path_with_working_dir: new happy-path -- absolute path with working_dir outside the server CWD resolves correctly.

All 152 tests pass. cargo clippy -- -D warnings and cargo fmt --check clean.

[aptu-dev]

Aptu Review

✅ Approve — This PR updates validate_path_relative_to to allow absolute paths when a working_dir is provided, resolving an issue where absolute paths were incorrectly rejected. It achieves this by bypassing validate_parent_in_root for new files and instead canonicalizing the parent directory to ensure it exists and is a valid directory.

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_{Posted by aptu}

[aptu-dev]

Aptu Review

✅ Approve — This PR updates validate_path_relative_to to correctly handle absolute paths when an explicit working_dir is provided. It replaces the overly restrictive validate_parent_in_root check with a direct resolution strategy that respects the operator-defined scope while maintaining security against path traversal via canonicalize.

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_{Posted by aptu}

[aptu-dev]

Aptu Review

✅ Approve — This PR updates validate_path_relative_to to correctly support absolute paths when an explicit working_dir is provided. It removes the restrictive validate_parent_in_root check for the require_exists=false case, shifting the responsibility of containment to the caller as per the MCP specification, while maintaining robust path resolution and error reporting.

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_{Posted by aptu}