fix(bedrock): preserve regional runtime on model switch

[BROCCOLO1D]

Summary

• Fixes mid-session /model switches to Bedrock regional inference profiles by preserving the resolved Bedrock region and SDK path.
• Closes [Bug]: Mid-session /model switch to Bedrock never sets _bedrock_region — regional inference profiles (jp.*/eu.*/apac.*) fail with ValidationException "The provided model identifier is invalid" #41296.

Why

• resolve_runtime_provider() accepted target_model but the Bedrock branch routed from the stale config default, so switching from a non-Bedrock default to Claude-on-Bedrock could select bedrock_converse incorrectly.
• switch_model() also never rebuilt the Bedrock-specific runtime state, leaving _bedrock_region unset and falling back to us-east-1 for regional jp.* / eu.* / apac.* profiles.

Changes

• hermes_cli/runtime_provider.py: route Bedrock dual-path selection from target_model when present.
• agent/agent_runtime_helpers.py: rebuild Bedrock switch state without constructing an OpenAI client, including _bedrock_region and AnthropicBedrock for Claude models.
• tests/hermes_cli/test_runtime_provider_resolution.py: cover target-model Bedrock routing for Claude and non-Claude models.
• tests/run_agent/test_switch_model_context.py: cover Bedrock Anthropic and Converse switch rebuild paths.

Validation

• python -m pytest tests/hermes_cli/test_runtime_provider_resolution.py tests/run_agent/test_switch_model_context.py -o 'addopts=' -q
• python -m ruff check agent/agent_runtime_helpers.py hermes_cli/runtime_provider.py tests/hermes_cli/test_runtime_provider_resolution.py tests/run_agent/test_switch_model_context.py

Scope

• In scope: mid-session Bedrock model-switch runtime selection and region preservation.
• Out of scope: live AWS Bedrock API calls or changes to startup-time Bedrock initialization.

[BROCCOLO1D]

Closing this as stale: there has been no activity or maintainer follow-up for over a week, and I don't want to leave inactive work open. Happy to reopen/rework if this becomes useful.