fix: preserve custom image dimensions in the SDK

[ch-z-hc]

Bug

sdk.image.generate({ prompt: 'A landscape', width: 1920, height: 1080, aspect_ratio: '1:1' }) still sends aspect_ratio: '1:1'. The SDK tries to clear it in the defaults, but merging the caller's parameters restores it. The API gives aspect_ratio priority, so the requested custom dimensions are ignored.

Fix

Apply the existing sizing decision after spreading the request parameters, matching the CLI's precedence. Keep ratio-only requests, dimension validation, other fields and the caller's object unchanged. No new dependency or API surface.

The HTTP-fixture regression inspects the actual serialized request: original code fails the conflicting-ratio case; patched code passes it plus dimensions-only and ratio-only controls.

Validation

• Image SDK tests: 14 passed on Windows. Windows and Linux typecheck, lint and CLI/SDK/declaration build pass; lint retains one existing speech-test warning.
• Two independent automated blind reviews: no findings.
• Full Windows run with a validation-only fetch preload: 590 passed / 12 failed; unchanged main in the same setup: 587 passed / the same 12 failed.
• Full Linux repeat: 599 passed / 3 failed; unchanged main: 596 passed / the same 3 failed (two unreachable-region tests time out, plus the FileSDK upload base-URL assertion). An earlier patched run had 600 passed / 2 failed because that upload test passed. These unrelated failures were not diagnosed or fixed here; this is not a claim that the full suite is green.

The preload rejects non-loopback fetches by default; test-owned mocks can override it. Linux uses temporary configuration and a process-local no-op URL opener. No repository assertions, timeouts or skip conditions were relaxed. No live model calls.

Implemented and tested with Codex assistance. Blind reviews were automated, not human approval.

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