bug: fix malformed date in Bedrock AssumeRole session name

[Lyqed]

Description: (required)

• getAssumedRoleCredentials builds the STS RoleSessionName from ${date.getFullYear()}${date.getMonth()}${date.getDay()}. getMonth() is zero-indexed and getDay() returns the day of the week, not the day of the month, so the session name is a malformed date: 202665 on 2026-07-10, and 202706 on 2027-01-02, which reads as a mid-2027 date.
• The session name is not cosmetic: it appears in CloudTrail events and in the AWS CUR line-item identity column for every Bedrock call made with the assumed role, so the malformed value lands in audit and billing data.
• Fixed to the unambiguous zero-padded YYYYMMDD form via date.toISOString().slice(0, 10).replace(/-/g, '').
• No behavior change beyond the string value: the session name is not part of the credential cache key (${awsRoleArn}/${awsExternalId}/${awsRegion}), and the new value passes the STS RoleSessionName charset/length constraints ([\w+=,.@-]{2,64}).

Tests Run/Test cases added: (required)

• Verified old vs new values:

  Jan 2, 2027  old: 202706  new: 20270102
  Jul 10, 2026 old: 202665  new: 20260710
  valid session name charset/length: true
  

  (There is no existing unit test coverage for src/providers/bedrock/utils.ts; happy to add a test if you can point me at the preferred harness for provider utils.)

Type of Change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)