fix(ecs-mcp-server): add path validation for user-supplied paths

[matthewgoodman13]

Fixes N/A

Summary

Changes

Add a shared path_validation module to ecs-mcp-server, mirroring the one in
eks-mcp-server.

validate_file_path() in utils/security.py only pattern-matched the raw string for
../ sequences, so an absolute path, a ~ prefix, or a symlink pointing at a user
configuration directory was accepted. prepare_template_files() also swallowed validation
errors whose message contained does not exist, so a rejected path could still be created.

The new validate_path():

• Expands ~ and resolves paths with os.path.realpath, collapsing .. and following
  symlinks before any check
• Rejects paths resolving inside a sensitive directory (~/.aws, ~/.ssh, ~/.kube,
  ~/.gnupg, ~/.docker, /etc, /root, /var/lib), and paths containing one, since a
  parent such as $HOME would pull it into a Docker build context
• Derives the home directory entries from both $HOME and the password database
• Accepts relative paths and returns the resolved absolute path

Applied in prepare_template_files (before os.makedirs and the template writes),
build_and_push_image (before the directory becomes the docker buildx build context), and
validate_cloudformation_template. validate_file_path is removed; every caller now uses the
new module.

Two differences from the EKS module: relative paths are accepted rather than rejected, and one
validate_path(path, must_exist=False) replaces two functions with identical bodies.

User experience

Before, app_path was used as written, so ~/.aws — or a symlink resolving to it — became
the template output directory and the Docker build context. Now such a path is rejected
before anything is written or built:

Path '~/.aws' resolves to '/home/user/.aws', which is not usable because it would
expose the sensitive directory '/home/user/.aws'

Project directories, absolute or relative, are unaffected.

Testing

787 tests pass, up from 722 on main. path_validation.py is at 100% line and branch
coverage, covering symlink, ~, relative, traversal, ancestor, prefix-collision and
case-insensitive-filesystem inputs. ruff check, ruff format, pyright, bandit and
uv build are clean.

Checklist

If your change doesn't seem to apply, please leave them unchecked.

• I have reviewed the contributing guidelines
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Is this a breaking change? (N)

Paths resolving inside or containing one of the listed directories are now rejected where they
were previously accepted. No other input is affected.

RFC issue number: N/A

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.15%. Comparing base (ebcafae) to head (c38994d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4480   +/-   ##
=======================================
  Coverage   93.14%   93.15%           
=======================================
  Files        1038     1039    +1     
  Lines       87530    87568   +38     
  Branches    14121    14124    +3     
=======================================
+ Hits        81529    81570   +41     
+ Misses       3638     3637    -1     
+ Partials     2363     2361    -2     

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