Cache rework to better use hed-schemas version manifest

[VisLab]

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[Copilot]

Pull request overview

This PR reworks HED schema caching/version discovery to use a repo-level schema_versions.json manifest from hed-schemas as a fast path, reducing GitHub REST API crawling and enabling single-version on-demand downloads.

Changes:

• Add hed.schema.schema_version_manifest to parse a global version manifest and build raw download URLs pinned to a specific repo commit.
• Update hed.schema.hed_cache to (a) use the manifest fast path in get_available_hed_versions, (b) seed caches with bundled schemas before attempting network downloads, and (c) download only a requested schema version on-demand.
• Expand unit/spec tests to cover manifest parsing, fallback behaviors, and single-version download behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
hed/schema/schema_version_manifest.py	New manifest parsing helpers for listing versions and locating per-version download metadata.
hed/schema/hed_cache.py	Integrates manifest fast path, improves cache seeding robustness, and adds single-version download helper.
tests/schema/test_schema_version_manifest.py	New unit tests for manifest parsing behaviors (no I/O).
tests/schema/test_hed_schema_io.py	Adds regression tests for fallback behavior and ensures manifest URL is treated as library-neutral in caching tests.
spec_tests/test_hed_cache.py	Updates spec test to verify on-demand downloading of a non-bundled library schema version.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.