Rewrite Rust unit tests to use explicit SchemaColumn fixtures (drop infer_test_schema_columns)

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Problem

#238 drops RegisteredModel.schema and the production JSON DDL path, but keeps infer_test_schema_columns and RegisteredModel::new_for_test as a pragmatic #[cfg(test)] bridge: JSON schema fixtures → minimal SchemaColumn → ModelCodecPlan::compile_from_columns.

That adapter is not production code, but it preserves a second JSON→IR inference path in the Rust test suite. Type-resolution behavior is already pinned on the lowering crate and cross-emitter parity tests; the adapter exists only for fixture ergonomics.

Proposed solution

1. Replace RegisteredModel::new_for_test(json!({...})) call sites in Rust unit tests (query.rs, operations.rs, codec_plan.rs, state.rs, etc.) with explicit SchemaColumn fixtures (or a small shared test_fixtures module of named column sets).
2. Delete infer_test_schema_columns, infer_test_logical_type, and the #[cfg(test)] JSON helpers they depend on (resolve_ref, property_json_type_and_format, json_schema_logical_type in schema.rs — only if no other test users remain).
3. Optionally introduce RegisteredModel::new_for_test_from_columns(columns, table_name) as the canonical test constructor.

Acceptance criteria

• No infer_test_schema_columns (or JSON→SchemaColumn inference) in the Rust codebase.
• RegisteredModel::new_for_test either removed or rewritten to take Vec<SchemaColumn> only.
• All existing Rust unit tests green (cargo test).
• No regression in pytest codec/hydration matrix (fixtures change is Rust-only).

Context

• Follow-up to: Drop RegisteredModel.schema — migrate ordering and dead JSON DDL path #238 (IR-only registration cleanup — keeps test adapter temporarily per grilling decision 2026-07-07)
• Epic: [IR-P9][WS6] compatibility cutover and shim removal (epic) #107 (Phase 9 / IR-first shim removal)
• Sequencing: land after Drop RegisteredModel.schema — migrate ordering and dead JSON DDL path #238 merges

Documentation impact

none — internal test infrastructure only.