Split block validation into HTML checks and live-editor auto-fix

[youknowriad]

Summary

• Replace the legacy validate_blocks flow with two tools: validate_html_blocks for core/html policy checks and validate_and_fix_blocks for live-editor validation with safe serialization fixes.
• Remove HTML policy enforcement from the core validator and return a proposed fixed block payload plus diff when editor serialization changes are needed.
• Update the agent instructions, tool registry, and prompt guidance to use the new two-step validation flow.
• Trim and refocus validation tests around behavior rather than legacy tool wiring.

Testing

• Focused AI tool and block-policy tests passed.
• Typecheck passed.
• ESLint fix/format was run on the touched files.

[wpmobilebot]

📊 Performance Test Results

Comparing 9293761 vs trunk

app-size

Metric	trunk	9293761	Diff	Change
App Size (Mac)	1339.26 MB	1339.27 MB	+0.01 MB	⚪ 0.0%

site-editor

Metric	trunk	9293761	Diff	Change
load	1741 ms	1781 ms	+40 ms	⚪ 0.0%

site-startup

Metric	trunk	9293761	Diff	Change
siteCreation	9600 ms	9598 ms	2 ms	⚪ 0.0%
siteStartup	4924 ms	4919 ms	5 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)