[Ruby] Blockless RepeatedField mutators return corrupting proxy enumerators

Summary

Blockless RepeatedField mutators such as delete_if, select!, collect!, and sort_by! return ProxyingEnumerator. Its each writes the block's predicate/transformation result directly into field slots, so predicate methods try to assign booleans into typed fields and transformation semantics diverge from Array.

Reproduction

On current main (5b1c20741838b8359193b97895cb0ff35b4ecf79) and 4.36.0, call field.delete_if without a block, then execute the returned enumerator with a predicate. An integer field attempts to accept true/false, producing a type error instead of deleting selected values.

Proposed correction

Return enum_for(method_name, *args) when the Array operation yields an Enumerator. When a block is later supplied, the wrapper re-invokes the real mutator and preserves its normal receiver/nil result contract; the broken 38-line proxy can be removed. Candidate: 40851eeb900925559ca730aa845be4301e746f36.

The isolated model covers the blockless mutator family and receiver/nil semantics. The cumulative candidate passes 332 native tests /556,948 assertions and 329 FFI tests /598,403 assertions with no failures or errors (documented omissions only).

I used an AI coding assistant during source review and verification. I am opening an issue first because CONTRIBUTING asks for tests and maintainer agreement; this dependency audit is not permitted to modify repository tests and no CLA was signed as part of it.

Start by locating the Ruby RepeatedField mutator implementations and the 38-line ProxyingEnumerator described in the issue. Compare blockless delete_if, select!, collect!, and sort_by! with Array enumerator behavior, then use the isolated model and native/FFI test suites to verify receiver and nil-result semantics. Done means blockless calls return a correct enumerator without corrupting typed fields.