fix: confine manifest paths to workspace and harden URL fetcher (#528)

* docs: add design for manifest path traversal fix

* docs: add implementation plan for manifest path traversal fix

* fix: confine manifest paths to workspace

moveFileToTmpDir previously used path.join(tempDirectory, originalFilepath),
which normalizes ../ sequences. A manifests input containing a traversal
sequence caused the action to read .yaml/.yml files from outside the
workspace and write copies outside RUNNER_TEMP. Directory inputs made
this stronger because recurisveManifestGetter enumerated YAML files
under the traversed directory.

Add assertPathWithinWorkspace, which resolves symlinks via realpathSync
and rejects any path not contained in GITHUB_WORKSPACE. Apply it in
getFilesFromDirectoriesAndURLs before lstat / readdir / file inclusion.
Rewrite moveFileToTmpDir to use a basename-only destination under
RUNNER_TEMP with a getCurrentTime() uniquifier to avoid collisions,
matching the safer pattern already used by getNewTempManifestFileName.

* fix: handle errors in writeYamlFromURLToFile

The https.get callback was marked async without any await, which caused
thrown errors to be silently swallowed as floating promise rejections.
There were no error listeners on the response stream or the file
writer, so socket or disk errors hung the promise instead of rejecting
it. On HTTP status >= 400 the function called reject but then fell
through and opened a write stream anyway.

Drop the misleading async, return after rejecting HTTP errors, drain
the response, and add error listeners on both streams. Wrap the string
verification error in new Error so stack traces are preserved.

* fix: harden verifyYaml and warn on unset GITHUB_WORKSPACE

Two follow-ups from review of the path-traversal series:

verifyYaml is called inside writeYamlFromURLToFile's finish listener.
If the body parsed to a null YAML document (e.g. "---" or a multi-doc
file with a trailing separator), the loop dereferenced obj.kind on
null and threw. Because the throw happened inside an EventEmitter
listener attached to a WriteStream rather than the Promise executor,
it was not routed to reject, so the promise hung. Wrap the finish
body in try/catch and add a null guard inside verifyYaml.

assertPathWithinWorkspace previously returned silently when
GITHUB_WORKSPACE was unset. In a real Action run the runner always
sets it, so unset is a signal that something is wrong with the
environment, not "skip the security check". Emit core.warning so a
misconfigured self-hosted runner does not lose the containment
protection without notice.

* test: realpath both sides of dirname comparison

On macOS, RUNNER_TEMP under /var/folders/... resolves through a
/private symlink. moveFileToTmpDir builds its destination from the
raw RUNNER_TEMP, so comparing path.dirname(out) directly with
realpathSync(tmpDir) would fail on macOS. Normalize both sides.

* fix: resolve relative manifest paths against workspace and clean up URL temp files

Address two Copilot review comments on PR #528:

- assertPathWithinWorkspace now resolves relative inputPath values
  against the realpathed GITHUB_WORKSPACE instead of process.cwd().
  Previously a step that changed CWD could cause unexpected
  rejections (or false acceptances) for relative manifests inputs.
  Absolute paths are still passed through and validated unchanged.

- writeYamlFromURLToFile now unlinks the partial temp file on any
  rejection that occurs after the write stream is created
  (writer error, response error, verification failure, sync throw
  in verify). The success path still resolves without unlinking.
  Pre-stream request errors leave nothing to clean up.

Tests added: a workspace-relative resolution test that deliberately
chdirs elsewhere, plus two cleanup-assertion tests covering
verification-failure and mid-stream response error.

Author: gambtho