fix: bound Action bundle manifest validation

[sylvesterkaczmarek]

Summary

Make the release-side Action bundle validator enforce the same 16 KiB manifest bound as the Action runtime.

Problem

The protected Action runtime reads bundle-manifest.json through readJsonBounded(..., 16 * 1024, ...), so a distribution manifest larger than 16 KiB is rejected before the Action can start.

script/validate-action-bundle, which is used to validate assembled/release bundle state, checks the manifest's structure and fields but currently reads it with an unbounded read_text. A manifest can therefore pass packaging validation and still be unusable by the runtime.

Evidence / reproduction

• action/lib.cts::validateBundle reads the bundle manifest with an explicit 16 KiB bound.
• script/validate-action-bundle validates the same action/bundle-manifest.json contract but on the base commit has no corresponding size check before json.loads(manifest_path.read_text(...)).
• The schema's semantic values are tightly constrained, but JSON permits arbitrary insignificant whitespace. That gives a minimal reproduction without changing any signed identity field: take a valid generated manifest and append more than 16 KiB of spaces.
• json.loads still parses that document to exactly the same object, so the base release-side validator accepts it.
• The Action runtime rejects the same file at its bounded-read boundary before validateBundle can complete.
• The added provenance regression copies a valid assembled bundle, appends only trailing JSON whitespace until the file exceeds 16 KiB, and requires the packaging validator to reject it.

This closes a validator/runtime contract gap rather than introducing a new limit.

Change

• require bundle-manifest.json to be non-empty and at most 16 KiB in script/validate-action-bundle
• add a focused regression to the existing Action bundle provenance test
• preserve all existing schema, checksum, symlink and binary-size validation

No manifest schema, release metadata, binary, or Action runtime behavior changes.

[GrantBirki]

The packaging limit now matches the Action runtime, and the regression test covers oversized manifests. Looks good, thanks!