Centralize variable lookup policies

[lovasoa]

Centralize request-variable lookup and merged enumeration in a private view. GET values remain borrowed, SET values release their RefCell borrow before returning, and explicitly NULL SET values suppress request fallbacks. Merged enumeration streams through Serde with SET > POST > GET precedence.

The existing $name lookup remains SET then GET, with the same POST deprecation warnings. Source-specific sqlpage.variables behavior and public Rust interfaces are unchanged.

Reuse existing regression structures:

• Extend test_variables_function and its SQL page with GET/POST array collisions, array lookups, missing and POST-only compatibility lookups, and a SET NULL phase. The same test checks merged and source-specific enumeration before SET, after SET, and after NULL assignment.
• Extend the existing immutable-variable fixture with NULL suppression and unchanged GET enumeration instead of adding a separate fixture.
• Keep one parameterized unit test for Missing versus explicit NULL, borrowed request scalars, owned SET scalars/arrays, and releasing the SET RefCell borrow before the returned value is consumed.
• Retain the run_sql isolation fixture covering inherited parent variables, child mutation, and explicit NULL input without changing the parent scope.

The variable-access unit test module shrinks from 93 lines to 44, and total PR additions decrease from 214 to 195. The consolidation changes tests only.

Validation: cargo fmt --all, formatting check, cargo clippy --all-targets --all-features -- -D warnings, and cargo test --features odbc-static passed (236 unit and 89 integration tests with in-memory SQLite). Local fixture servers required running tests outside the network sandbox.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-07T10:20:49.685033Z	da7cf0d	New commits

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