refactor: Compact schema_generation to <150 LOC

[MQ37]

Context

Reviewer ceiling for src/utils/schema_generation.ts + tests is 150 LOC of code (comments / blank lines free). The fix in #854 lands at 387.

Solution

Compact both files to 148 combined LOC with the same behavior. Stacked on top of #854 so the diff in this PR is purely the compression — no behavior change to review here.

File	#854	This PR
src/utils/schema_generation.ts	122	69
tests/unit/schema_generation.test.ts	265	79
Combined	387	148

Worth your attention

• Coverage equivalent, fewer test blocks. 28 → 21 tests. The 6 type-union cases and 5 format-detection cases are folded into two it.each tables. Same regression coverage: NYC sushi case, heterogeneous keys, format false-positive guard.
• Two ! non-null assertions added in merge. Both inside branches guarded by &&/|| predicates immediately above (ap[k] && bp[k] and a.items || b.items). The narrowing alternative was 4 LOC longer.
• Internal types dropped, public surface unchanged. JsonSchemaArray, JsonSchemaObject, SchemaGenerationOptions, and the removeEmptyArrays export were all unused outside the module. JsonSchemaProperty stays (imported by actor_execution.ts).

Open

• If you prefer fix: Merge dataset item schemas across heterogeneous shapes #854's verbose version, close this PR. If you prefer this one, squash-merge it as a replacement for fix: Merge dataset item schemas across heterogeneous shapes #854 (or merge fix: Merge dataset item schemas across heterogeneous shapes #854 first then this on top).