refactor: rename Bedrock credential fields to generic names (#703)

Rename AWS-specific field names in ProviderSettingsSchema to generic
names that could be reused by other providers:
- awsAccessKeyId → accessKey
- awsSecretAccessKey → secretKey
- awsSessionToken → sessionToken
- awsRegion → region

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
(cherry picked from commit 185d40875649f8e2bab2a4f962cc6dd8f8e89606)