fix(sagemaker): Validate clusterId format in open-notebook extension

[aakashmandavilli96]

Issue

D316398873

Description of Changes

Adds input validation for the clusterId URL parameter in the
sagemaker-open-notebook extension. Previously only region was validated
(via isValidRegion); this change adds an equivalent isValidClusterId
check so clusterId is restricted to alphanumeric characters, hyphens, and
underscores before it is used to generate notebook cell content. Malformed
values are rejected with a user-facing error.

Since extension.ts is generated at build time from the quilt patch series,
the change is made in patches/sagemaker/sagemaker-open-notebook-extension.diff
and regenerated with quilt refresh (no other files change).

Testing

• Regenerated the patched source via scripts/prepare-src.sh; the full patch
  series applies cleanly and the generated extension.ts contains the new
  isValidClusterId validator and the guard in loadAndDisplayNotebook.
• Type-checked the edited extension with tsc --noEmit; no errors on the
  change (only environmental module-resolution messages from the uninstalled
  toolchain).
• Confirmed valid IDs (e.g. ml-cluster-123) are accepted and values
  containing shell metacharacters, spaces, or newlines are rejected.

Screenshots/Videos

N/A

Additional Notes

Minimal, low-risk input-validation change that mirrors the existing
isValidRegion pattern already in the extension.

Backporting

This change applies to the other active version branches. Separate PRs will be
raised for main, 1.2, and 1.0.

---

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