feat(providers): add LLM sampling controls

[deepujain]

Summary

• add optional SKILLSPECTOR_TEMPERATURE sampling control for hosted providers
• add optional SKILLSPECTOR_SEED forwarding for OpenAI-compatible and Azure OpenAI providers
• validate configured values before model construction and preserve provider defaults when unset or blank
• document provider support and the limits of reproducibility controls

This addresses the sampling-control portion of #334. Provider/model attribution in generated reports is intentionally unchanged because that requires a separate report-schema decision.

Validation

• pytest tests/unit/test_providers.py tests/unit/test_anthropic_proxy_provider.py tests/unit/test_new_providers.py tests/unit/test_bedrock_provider.py tests/unit/test_constants.py -q — 176 passed, 9 skipped
• pytest -m 'not integration and not provider' tests/ -q — 2804 passed, 13 skipped, 38 deselected, 4 xfailed
• ruff check src tests
• ruff format --check src tests
• git diff --check

Risk

• unset or blank controls preserve existing provider-default request behavior
• temperature is constrained to the portable 0..1 range; seed support remains provider/model dependent
• Anthropic, Anthropic proxy, and Bedrock receive temperature but never the OpenAI-specific seed
• CLI providers are unchanged because their subprocess interfaces own sampling behavior

[rng1995]

[SkillSpector Review]\n\nApproved. Sampling controls are validated centrally and mapped appropriately across OpenAI-compatible, Azure, Anthropic, Bedrock, and proxy providers, with unsupported seed parameters omitted where required. Current required checks pass.

[deepujain]

Resolved the conflict with current main by retaining both the merged output-language configuration and this PR’s sampling controls. Rebased to signed head 705bc1d; focused provider tests passed (176 passed, 9 skipped), the contributor suite passed (2,833 passed, 13 skipped, 38 deselected, 4 xfailed), and repository-wide Ruff checks are clean. GitHub checks are rerunning; ready for re-review.