feat: add AWS Bedrock provider support (#701)

* feat: add AWS Bedrock provider support

Add AWS Bedrock as a new AI provider with support for Claude and
Amazon Nova models. Uses @ai-sdk/amazon-bedrock for model integration
with AWS Access Key + Secret + Region credential configuration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: require awsRegion in Bedrock provider availability checks

Without this check, the Bedrock provider would appear as available in the
UI even when the AWS region is not configured, leading to runtime failures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address Bedrock security and config issues

- Scrub AWS credentials from settings export (security)
- Remove awsRegion requirement from availability checks (fallback to default)
- Preserve AWS credentials in provider import/merge

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
(cherry picked from commit 3995fc2c866a2262ac4b9eb523985f5fb40be7b5)