fix(bedrock): add live model discovery and region resolution for non-US regions

provider_model_ids("bedrock") fell through to a static _PROVIDER_MODELS
table containing only hardcoded us.* model IDs.  Users configured for
non-US AWS regions (eu-central-1, ap-northeast-1, etc.) saw wrong or no
models in /model and autocomplete.

Root causes fixed:

1. models.py: provider_model_ids() now calls discover_bedrock_models()
   keyed by the resolved region before falling back to the static table.
   A new bedrock_model_ids_or_none() helper in bedrock_adapter.py
   consolidates the discover -> extract IDs -> fallback pattern used by
   all three call sites.

2. providers.py: registers bedrock in HERMES_OVERLAYS with
   transport=bedrock_converse and auth_type=aws_sdk so
   get_provider("bedrock") and resolve_provider_full("bedrock") work.

3. model_switch.py: list_authenticated_providers() sections 2 and 3
   detect AWS credentials via has_aws_credentials() for aws_sdk
   overlays and use live discovery for the model list.

4. bedrock_adapter.py: resolve_bedrock_region() reads the configured
   region from botocore.session before falling back to us-east-1,
   covering users who set their region in ~/.aws/config via a named
   profile rather than env vars.

5. tui_gateway/server.py: passes provider= to get_model_context_length()
   so context window lookups work correctly for the Bedrock provider.

tui_gateway/server.py

@@ -2407,6 +2407,7 @@ def _(rid, params: dict) -> dict:
                     getattr(agent, "model", "") or _resolve_model(),
                     base_url=getattr(agent, "base_url", "") or "",
                     api_key=getattr(agent, "api_key", "") or "",
+                    provider=getattr(agent, "provider", "") or "",
                 )
                 ctx = preprocess_context_references(
                     prompt,