feat: add Requesty as a model provider

[devin-ai-integration]

What does this PR do?

Adds Requesty as a bundled model-provider plugin. Requesty is a unified LLM gateway for 300+ models with intelligent routing, caching, and failover — fully OpenAI-compatible at router.requesty.ai/v1.

Type of Change

• ✨ New feature (non-breaking change that adds functionality)

Changes Made

• plugins/model-providers/requesty/__init__.py — ProviderProfile with base URL, env vars, fallback models, and attribution headers
• plugins/model-providers/requesty/plugin.yaml — plugin manifest
• tests/providers/test_plugin_discovery.py — bump expected provider count from 33 → 34, add requesty to spot-check list

How to Test

1. python -c "from providers import get_provider_profile; p = get_provider_profile('requesty'); print(p.name, p.base_url)"
2. python -m pytest tests/providers/ -x -q — all 92 tests pass
3. Alias lookup: get_provider_profile('requesty-ai') resolves to the same profile

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes
• I've tested on my platform: Ubuntu (CI)

Documentation & Housekeeping

• I've updated relevant documentation — N/A (auto-wired via plugin system)
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact — N/A (pure Python, no platform-specific code)
• I've updated tool descriptions/schemas if I changed tool behavior — N/A

Link to Devin session: https://app.devin.ai/sessions/cc1ddf8ba6484d689ee8b50a2f09fb44
Requested by: @requesty-JohnCosta27

[devin-ai-integration]

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