[BUG] Version string is accepted but never validated against installed manifest

[ombagal777]

Description

The version field in the execution request is required and validated by Zod as a non‑empty string, but the server never checks whether this version exists in the installed language manifest. A request can specify an arbitrary version (e.g., python3 @ 99.0), and the engine will still run whichever version is currently installed, while echoing the requested version back in the response.

Impact

• Callers cannot rely on the version they requested actually being used.
• Silent mismatch between requested version and executed runtime can cause subtle behavior differences, especially for languages with fast‑moving ecosystems (Python, Node, Rust).
• Makes it hard to do reproducible, pinned builds or judge runs.

Expected Behavior

• The server validates the requested version against the installed manifest for the given language.
• If the version is not installed or not defined in the manifest, the request should fail with a clear error (e.g., HTTP 400 + structured error code).

Actual Behavior

• The server accepts any non‑empty version string.
• Execution uses whatever version is installed, and the response still echoes the user‑provided version.

Possible Solution

• Load the language manifest / langMeta for the requested language.
• Validate that payload.version matches one of the installed versions for that language.
• If not found, return a validation error such as:
  • {"error": "VERSION_NOT_INSTALLED", "details": { "language": "python3", "version": "99.0" }}
• Optionally, include a list of available versions in the error response to improve DX.