## Why
An explicit `service_tier = "flex"` could be dropped when fast mode was disabled or the model catalog did not advertise Flex, even though Flex is an API request option.
## What changed
- Preserve configured `flex` through core session settings, TUI tier resolution, and request construction regardless of fast-mode or catalog support.
- Omit `service_tier` from Amazon Bedrock requests, including with custom catalogs, because Bedrock only supports the implicit default tier.
## Testing
Add regression coverage for Flex configured at startup or through thread settings, connected thread startup, and TUI turn submission. Update review coverage to expect Flex with fast mode disabled, and verify that both Bedrock providers omit `service_tier`.
GitOrigin-RevId: 98f2761bdd780603839ebfec75cb983a5d55e554
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