AWS Bedrock as agent model backend

**Motivation**

No way to point Buzz agents at models served through **Amazon Bedrock**. Teams running Buzz self-hosted in AWS want this to draw down Bedrock credits, keep inference inside their AWS account (data residency), and reach the model families Bedrock hosts without depending on a single vendor's public API. We hit it deploying Buzz internally: we have AWS credits and a data-residency constraint, but the only path to Bedrock today is an external proxy.

**Proposed solution**

A first-class **Bedrock provider**, selectable per agent like the others, with:
- AWS **region** (+ cross-region inference profiles),
- **credential source**: IAM role, SSO profile, or access keys — via **SigV4**, not a bearer key,
- **model ID** selection, with live listing where account access allows.

Ideally preserving native features (e.g. Anthropic prompt caching) the way the Databricks path in #3463 does.

**Alternatives considered**

- **Custom raw base-URL provider** (#2749): can't work — bearer-token only, while Bedrock needs SigV4 signing and region-scoped model IDs (and it's reported broken).
- **LiteLLM/OpenAI-compat proxy → Bedrock**: works but adds a service to run and usually drops provider-native features.
- **Call the Anthropic API directly**: defeats the cost + data-residency goals.

**Additional context**

Prior art: **#3463** added Anthropic prompt caching for **Databricks** — Buzz already models enterprise LLM gateways, and Bedrock is the AWS-native analog.
Duplicate check: none found (searched open + closed issues and PRs).

Start by reviewing Buzz's existing model provider implementations and the Databricks path from #3463 to understand how enterprise gateways and native features are represented. Define the Bedrock provider around region and cross-region profiles, IAM/SSO/access-key credential sources with SigV4, model IDs, and optional live listing; done means agents can select it and use Bedrock without an external proxy.

Written by the indexing model from the issue text.