fix: log beta headers on 'invalid beta flag' error for faster diagnosis

[AmarkFox]

Problem:
When Bedrock rejects a request with 'invalid beta flag', the error message does not indicate which specific beta header caused the failure. Diagnosing the issue requires manually searching CloudWatch logs for 'Passing through beta header' entries and cross-referencing timestamps, which is slow and error-prone.

This is important because the proxy uses a passthrough-by-default strategy for unknown beta headers (to maximize feature compatibility), so new unsupported headers from Claude Code upgrades will occasionally cause this error until they are added to the blocklist.

Change:
Add targeted logging in both InvokeModel and InvokeModelWithResponseStream error handlers: when the error message contains 'invalid beta', log the full list of beta headers that were sent to Bedrock, plus the original anthropic-beta header from the client.

Example log output on error:
[ERROR] Code: ValidationException
[ERROR] Message: invalid beta flag
[ERROR] Beta headers sent: ['interleaved-thinking-2025-05-14', 'bad-header']
[ERROR] Original anthropic-beta header: interleaved-thinking-2025-05-14,bad-header,...

Diagnosis workflow:

1. User reports 'invalid beta flag' error from Claude Code
2. Search CloudWatch: filter-pattern '"invalid beta"'
3. Log shows exactly which headers were sent
4. Add the offending header to blocklist via Admin Portal (Beta Headers management page, no redeployment needed)

Issue #, if available:

Description of changes:

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