docs: add Gemini as a fifth LLM provider across all docs

[agarvin2002]

Update generation.md, api-reference.md, development.md, and README to document GeminiProvider alongside the existing four providers, including the thinking_budget=0 requirement for Gemini 2.5 Flash.

What and Why

Type of Change

• Bug fix
• New feature
• Refactor / clean-up
• Performance improvement
• Docs / comments
• CI / tooling / infra

Changes Made

Testing Done

Checklist

Code quality

• uv run pytest tests/unit/ passes locally
• uv run ruff check . returns no errors
• New code has corresponding unit tests using fakes, not unittest.mock.patch

Architecture rules

• No Django imports inside ingestion/, retrieval/, generation/, or agents/
• New dependencies injected via constructor — not imported as module-level globals inside functions
• New LLM-calling code uses the existing LLMProviderPort / StructuredLLMPort — no direct SDK calls in views or tasks

Agent pipeline (skip if not touching agents/)

• Node functions catch all exceptions, set state["error"], and return normally — no unhandled raises
• New routing functions check state.get("error") before choosing the next node

HTTP layer (skip if not adding/changing endpoints)

• New endpoints have a ThrottleClass applied
• New endpoints return {"detail": "..."} for all error responses via the existing exception hierarchy
• Rate limit constant added to core/constants.py if a new throttle tier was needed

Data layer (skip if not touching models)

• Migrations generated and committed (uv run python manage.py makemigrations --check passes)
• New VectorField columns have an HNSW migration with atomic = False

Security

• No secrets, API keys, or credentials in code or comments
• User-supplied input is validated by a serializer before reaching service or pipeline code
• File uploads validated for type and size before storage

Breaking Changes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 3be0ec8f-b9bb-4a08-9371-e5153e3a7a8e

📥 Commits

Reviewing files that changed from the base of the PR and between bfb7de4 and e511577.

📒 Files selected for processing (4)

• README.md
• docs/api-reference.md
• docs/development.md
• docs/generation.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/gemini-provider

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