fix: Potential fix for code scanning alert no. 13: Uncontrolled data used in path expression

[skyoo2003]

Potential fix for https://github.com/skyoo2003/devcloud/security/code-scanning/13

To fix this safely without changing functionality, harden safePath in internal/services/s3/store.go so it:

1. Resolves both baseDir and candidate path to absolute paths (filepath.Abs).
2. Uses filepath.Rel(baseAbs, candidateAbs) to verify containment.
3. Rejects paths where rel == ".." or starts with ".."+separator (escape).
4. Keeps returning normalized absolute path for downstream file ops.

This is the best single fix because it preserves current API behavior and all call sites, while replacing a fragile prefix check with a standard, cross-platform path containment validation pattern. No provider-side routing changes are required for this specific sink hardening.

Suggested fixes powered by Copilot Autofix. Review carefully before merging.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Hardens the FileStore.safePath implementation to use absolute paths and filepath.Rel for robust directory containment checks, addressing a path traversal/code scanning alert without changing the public API.

File-Level Changes

Change	Details	Files
Strengthen path traversal protection in FileStore.safePath using absolute paths and relative path containment checks.	Resolve fs.baseDir to an absolute path at the start of safePath and handle resolution errors explicitly. Join path components using the absolute base directory instead of the raw baseDir value. Normalize the joined path and resolve it again to an absolute candidate path, returning a wrapped error if resolution fails. Use filepath.Rel between the absolute base directory and the candidate path to determine whether the candidate escapes the base directory. Treat any relative result equal to ".." or starting with ".." plus the OS path separator as a traversal attempt and return a descriptive error including the candidate path. Return the normalized absolute candidate path instead of the previously returned cleaned path to keep downstream file operations on absolute, validated paths.	internal/services/s3/store.go

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[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• Consider resolving fs.baseDir to an absolute path once in NewFileStore and storing it on the struct, rather than calling filepath.Abs on every safePath invocation to avoid repeated filesystem/OS work on hot paths.
• Since joined is already built from baseAbs, calling filepath.Abs on cleaned is redundant; you can simplify by dropping the second Abs call and using cleaned directly as the candidate path for Rel and the return value.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider resolving `fs.baseDir` to an absolute path once in `NewFileStore` and storing it on the struct, rather than calling `filepath.Abs` on every `safePath` invocation to avoid repeated filesystem/OS work on hot paths.
- Since `joined` is already built from `baseAbs`, calling `filepath.Abs` on `cleaned` is redundant; you can simplify by dropping the second `Abs` call and using `cleaned` directly as the candidate path for `Rel` and the return value.

## Individual Comments

### Comment 1
<location path="internal/services/s3/store.go" line_range="41-45" />
<code_context>
+		return "", fmt.Errorf("resolve candidate path: %w", err)
+	}
+
+	rel, err := filepath.Rel(baseAbs, candidateAbs)
+	if err != nil {
+		return "", fmt.Errorf("resolve relative path: %w", err)
+	}
+	if rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
+		return "", fmt.Errorf("path traversal detected: %s", candidateAbs)
 	}
</code_context>
<issue_to_address>
**🚨 issue (security):** Path traversal check does not account for symlinks inside the base directory.

`Abs` + `Rel` blocks `..` traversal but not symlinks under `baseAbs` that point outside it, so the isolation guarantee can be broken. For stronger confinement, resolve symlinks (e.g. with `filepath.EvalSymlinks`) on `baseAbs` and/or `candidateAbs` and run the `Rel` check on those resolved paths.
</issue_to_address>

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