[#3583333] Revert render-time validation from processSchemaValues

Removes both assertValidPatternContent calls introduced by #3563760 (commit b5a22725) from the PatternFieldProcessorPluginManager render path. These calls caused two regressions on upgrade from 9.1.2:

1. WARNING log spam on every cache-miss render for blocks whose stored values did not satisfy strict schema constraints (e.g. type mismatches from content saved before validation was enforced).

2. Visitor-visible "This content is unavailable." errors for blocks containing schema-invalid stored content, because the hard post-processing validation threw SchemaValidationException and interrupted rendering.

assertValidPatternContent and PatternValidationHelper are retained for use by PLATCMS-3245 (stamp-aware render policy), which will wire controlled validation back onto the render path. Tests are updated to assert 9.1.2 render-parity rather than the regression behavior.

Closes #3583333