UB: Never expiring creds bug

[okruitho]

Issue #, if available:

Description of changes:
Fix non-expiring CRT credentials being incorrectly reported as expired by CrtCredentialsProvider.

CRT documents UINT64_MAX as “no expiration”. Previously, the provider converted this sentinel into a DateTime as though it were an ordinary timestamp, exceeding the representable range and causing IsExpired() and IsExpiredOrEmpty() to return true.

For gcc this works fine; when using clang the casts from uint64_t to double to uint64_t give undefined behaviour, depending on compliler optimisation.

Downstream impact

We encountered this in DuckDB’s AWS extension, which uses this check before copying credentials into an S3 secret. Our toolchain is clang based. The incorrect expiration status caused valid credentials to be omitted, surfacing in its MinIO integration test.

Changes

• Preserve AWSCredentials’ default non-expiring value when CRT returns UINT64_MAX.
• Use the uint64_t seconds constructor for finite expiration timestamps, avoiding the floating-point conversion.
• Add regression coverage for non-expiring credentials, ensuring all three state checks return false and subsequent retrieval uses cached credentials.
• Add coverage confirming finite expiration timestamps are preserved.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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