Add MiniMax text-to-speech support

[octo-patch]

Reason: Add native MiniMax text-to-speech support through the existing media provider route.

• Send regional /t2a_v2 requests with the current default model, speech options, and hex or URL response handling.
• Route minimax/ audio models to the media provider while preserving provider validation errors.
• Add focused coverage for request construction, regional endpoints, response parsing, validation, and high-level routing.

Checks:

• uvx --from ruff==0.15.22 ruff check .
• uvx --from ruff==0.15.22 ruff format --check agentfield/media_providers.py agentfield/agent_ai.py tests/test_minimax_audio.py tests/test_minimax_video.py
• uv run --frozen --extra dev ./scripts/run_pytest.sh tests/test_minimax_audio.py tests/test_minimax_video.py tests/test_media_providers.py tests/test_agent_ai.py --no-cov

[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	87.00%	87.40%	↓ -0.40 pp	🟡
sdk-go	92.50%	92.00%	↑ +0.50 pp	🟢
sdk-python	93.82%	93.73%	↑ +0.09 pp	🟢
sdk-typescript	91.05%	90.42%	↑ +0.63 pp	🟢
web-ui	84.76%	84.79%	↓ -0.03 pp	🟡
aggregate	85.59%	85.75%	↓ -0.16 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.