Fix path traversal bypass in ResolveLocalPath sandbox

[nishantmonu51]

ResolveLocalPath is used as the file/DuckDB sandbox in hosted deployments (via AllowHostAccess=false). Two defects let a project-supplied path escape the repo root:

• Absolute inputs were returned without filepath.Clean, so .. segments survived (filepath.Join, which cleans, only ran for relative paths). path: /repo/../../etc/passwd passed HasPrefix("/repo/../../etc/passwd", "/repo") and resolved to /etc/passwd.
• The prefix check had no trailing separator, so a sibling directory sharing the root's prefix passed. With root /data/proj123, path: ../proj123-x/secret cleaned to /data/proj123-x/secret and slipped through — a potential cross-tenant read.

Fix:

• filepath.Clean the resolved path for both the absolute and relative branches before the sandbox check.
• Replace strings.HasPrefix(finalPath, root) with finalPath == root || strings.HasPrefix(finalPath, root+separator), cleaning root first.
• Added TestResolveLocalPath covering absolute/relative traversal, both sibling-prefix variants, in-root paths, and the allowHostAccess=true passthrough.

Reproducing steps:

1. Configure a duckdb or local file connector with allow_host_access: false (the state hosted Rill Cloud runs the sandbox in).
2. Create a source/model whose path is an absolute path containing .., e.g. path: /<project_root>/../../etc/hosts.
3. Reconcile/ingest the source. Before this change the file resolves outside the project root and is read; after it, the request is rejected with "path is outside repo root".
4. Alternatively, with project root /data/proj123, point path at ../proj123-x/<file> and observe it previously resolved to a sibling tenant directory that still passed the prefix check.

In code, the regression is captured by:

• ResolveLocalPath("/root/../etc/x", "/root", false) — now returns an error.
• ResolveLocalPath("../root-x/secret", "/root", false) — now returns an error.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!