docs(ai-sessions): Vertex AI provider + first-class ai_agent bedrock/vertex blocks

[maximelb]

Summary

• Adds a Google Cloud Vertex AI section to `docs/9-ai-sessions/alternative-providers.md`, mirroring the existing Amazon Bedrock guidance (model IDs, IAM, region, configuration examples).
• Documents the new first-class `bedrock` and `vertex` blocks on `ai_agent` Hive records (introduced in legion_config_hive#287).
• Keeps the existing `environment`-based configuration documented for interactive Profiles and inline D&R `profile:` blocks where the structured fields don't apply.

Pairs with

• legion_config_hive#287 — adds the schema and validator.

Test plan

• Render `mkdocs serve` locally and confirm the new Vertex section + updated Bedrock section render correctly with no broken links.
• Cross-check model IDs, region prefixes, and env var names against the upstream Anthropic docs (`docs.claude.com/en/docs/claude-code/google-vertex-ai`, `docs.claude.com/en/docs/claude-code/amazon-bedrock`).
• Verify the `hive://secret/` reference convention matches what the consumer actually accepts before flipping out of draft.

🤖 Generated with Claude Code