test(aws): Make integration test cassettes AWS region agnostic

[Michael Chin (michaelnchin)]

Fixing some cassette errors seen in live integ test runs following the updates in #1154:

E               vcr.errors.CannotOverwriteExistingCassetteException: Can't overwrite existing cassette ('tests/cassettes/test_thinking[v0].yaml.gz') in your current record mode (<RecordMode.ONCE: 'once'>).
E               No match for the request (<Request (POST) https://bedrock-runtime.***.amazonaws.com/model/us.anthropic.claude-sonnet-5/converse-stream>) was found.
E               Found 1 similar requests with 1 different matcher(s) :
E               
E               1 - (<Request (POST) https://bedrock-runtime.us-west-2.amazonaws.com/model/us.anthropic.claude-sonnet-5/converse-stream>).
E               Matchers succeeded : ['method', 'body']
E               Matchers failed :
E               uri - assertion failure :
E               https://bedrock-runtime.***.amazonaws.com/model/us.anthropic.claude-sonnet-5/converse-stream != https://bedrock-runtime.us-west-2.amazonaws.com/model/us.anthropic.claude-sonnet-5/converse-stream

Basically, the new cassettes for the revised tests with Sonnet 5 were recorded from my local runs on us-west-2, while the CI actually runs on a different region (that's obfuscated by the AWS_REGION secret). VCR's default matcher requires the endpoint URIs to match exactly, so the replays fail.

To fix this, I've loosened the URI check by replacing VCR's default matcher with a custom region_agnostic_uri matcher, stripping the region ID from the host before performing the comparison.

This has two advantages:

• It allows future cassettes to be recorded from any region.
• We can easily move integ CI to a different AWS_REGION without having to update the cassettes to do so.