fix: replace inline python3 -c blocks and bare heredocs with run: | block scalars in two workflows

[TMHSDigital]

Fixes #5. Same root cause class as the CFX YAML quoting cluster (TMHSDigital/CFX-Developer-Tools#5, #6, #7, fixed in CFX PR #8).

What was broken

Unity had two distinct flavors of the same YAML scanner bug, both causing the workflow files to be rejected before any job ran:

1. 3 multiline python3 -c "..." blocks in validate-plugin-manifest (validate.yml lines 100-130). Same bug as CFX#5 — embedded for n in data: and assert ...: colons terminate the YAML implicit scalar.
2. 7 python3 << 'PYEOF' blocks with python body at column 0 and no run: | block-scalar marker. The run value was parsed as the single-line plain scalar python3 << 'PYEOF', then the column-0 python body was treated as top-level YAML tokens and produced ScannerError: while scanning a simple key, could not find expected ':'.

What this PR does

Wraps every offending run in run: | literal block scalar with the python heredoc body indented to match. Heredoc delimiter PYEOF preserved for consistency with the original file.

• validate.yml: 9 blocks fixed (1 huge MCP data validator + 3 plugin manifest checks + skills/rules/snippets/templates/counts validators).
• update-unity-api.yml: 1 block fixed (the validate-updated-data step).

Net diff: 2 files changed, 266 insertions, 256 deletions (mostly indentation changes — the python content itself is untouched apart from leading whitespace).

Local verification

• Both files parse cleanly under PyYAML safe_load with newlines preserved in every script body (10 blocks total).
• Every python validation block runs successfully against the actual repo data:
  • 5 MCP data JSONs valid: unity_api_common.json (59 entries), deprecated_patterns.json (17), lifecycle_order.json (25), shader_properties.json (6), platform_defines.json (9).
  • plugin manifest schema valid (name kebab-case, version semver, author has name, keywords array).
  • All 18 skills pass frontmatter checks.
  • All 8 rules pass frontmatter checks.
  • All 20 snippets non-empty.
  • All 5 templates have README.md + at least one .cs file.
  • validate-counts: 18 skills, 8 rules, 20 snippets, 5 templates — all match plugin.json and README.md.
• Representative heredocs exercised end-to-end via bash to confirm the heredoc-to-python pipe works at runtime.

So once this lands, the validate workflow should go green for the first time since it last ran. The scheduled update-unity-api.yml will actually run its validation step when next triggered (Mondays at 06:00 UTC).

Context

Surfaced during ecosystem audit on 2026-04-25 (TMHSDigital/Developer-Tools-Directory#1 close-out). Same fix pattern applied here as in CFX PR #8 fifteen minutes earlier.