Add MiniMax image generation support

[octo-patch]

Reason: Add native MiniMax image generation support to the existing media provider pipeline.

Changes:

• Implement image generation requests with global and China regional endpoint support.
• Support URL and base64 image responses, optional request fields, and model-prefix routing.
• Add focused tests for request construction, response parsing, validation, and provider registration.

Checks:

• git diff --check origin/main...HEAD

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	9.0 KB	-	0.32 µs	-9%	✓	✓

✓ No regressions detected

[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 84%, aggregate ≥ 85%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	87.10%	87.40%	↓ -0.30 pp	🟡
sdk-go	92.70%	92.00%	↑ +0.70 pp	🟢
sdk-python	93.82%	93.73%	↑ +0.09 pp	🟢
sdk-typescript	91.17%	90.42%	↑ +0.75 pp	🟢
web-ui	84.75%	84.79%	↓ -0.04 pp	🟡
aggregate	85.62%	85.75%	↓ -0.13 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	0	—	➖ no changes
sdk-go	0	—	➖ no changes
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

✅ Patch gate passed

Every surface whose lines were touched by this PR has patch coverage at or above the threshold.

[AbirAbbas]

Last of the three MiniMax PRs — took these over since they'd been quiet for a few days.

I found one real bug here. Response parsing read data.image_urls unconditionally, but MiniMax documents base64 responses under data.image_base64. So response_format="base64" — one of the two modes this PR advertises — would have failed against the real API every time with "returned no image_urls". The reason it looked fine is that the base64 test built its fixture with an image_urls payload, so it asserted the implementation's assumption rather than the provider's actual contract, and passed while masking the defect.

I've pushed a fix that selects the response key from the requested format and names the actual mode in the error, plus corrected the base64 fixture to the documented image_base64 shape. I confirmed it fails before the change and passes after.

Two smaller things in the same commit: base_resp and data were only type-checked at the top level, so a non-dict nested value leaked AttributeError instead of the normalized RuntimeError (#852 already did this properly, so I matched its idiom), and I added regression coverage for the branches that had none — empty model, HTTP status errors, non-dict JSON, missing and empty image collections.

On the test_minimax_video.py edit: I checked it isn't weakening video coverage. It only drops the obsolete generate_image NotImplementedError expectation, which this PR makes untrue.

Since this landed last of the three, I rebased it and reconciled the overlap with #852 and #854 by hand: supported_modalities is now the union ['video', 'music', 'audio', 'image'], the H3 constants and video_model_metadata from #854 are preserved, and both stale NotImplementedError assertions are gone. Full Python SDK suite is green on the reconciled result.