Track model conversion correctness after #295 rollback

[jbeckwith-oai]

Context

PR #375 intentionally reverts #295 to restore the published v0.78 raw-request / parsed-response BaseModel contract before redesigning nested coercion. The exact rollback also restores several pre-#295 converter behaviors that deserve independent investigation and regression coverage.

These concerns must not be fixed by widening global coercion or changing BaseModel#[] / #to_h away from raw caller-owned values. Each should be reproduced against the published contract and solved narrowly.

Follow-up behaviors

• Isolate conversion errors between union variant attempts so a failed earlier variant cannot poison a later successful match (for example Realtime max_output_tokens: :inf).
• Preserve explicitly allowed nil values without recording a conversion error; distinguish optional (omittable) from nil?: true (nullable) deliberately.
• Preserve the incoming strictness state across the discriminated-union fast path before evaluating later fields.
• Return an already-coerced instance of the target BaseModel unchanged, retaining identity and out-of-band state such as last_response.
• Retain collection element conversion failures deterministically across ArrayOf and HashOf, independent of element order.
• Make nullable ArrayOf / HashOf matchers agree with their coercion and generated type contracts.

Required coverage

For each behavior, add a minimal runtime-level regression and at least one representative generated-model test where applicable. Include order permutations for collection/union error state, symbol/string discriminator forms where relevant, and assertions covering raw request access separately from parsed response materialization.

Related

• fix: coerce nested BaseModel fields #295 introduced the global coercion change now being rolled back.
• revert: nested BaseModel coercion (#295) #375 is the standalone rollback and intentionally does not solve these follow-ups.
• Unknown Stream Events Classified as a ResponseErrorEvent #287 remains the separate user-facing unknown-stream-event issue.