Instant + Duration overflow panic in elapsed-time retry and polling policies

Unconfirmed. This is the result of a search with Claude, may be a false positive.

In `src/gax/src/retry_policy.rs` and `src/gax/src/polling_error_policy.rs`, deadline calculations compute `state.start + self.maximum_duration` without checked arithmetic:

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/gax/src/retry_policy.rs#L475-L478

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/gax/src/retry_policy.rs#L497-L500

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/gax/src/retry_policy.rs#L514-L517

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/gax/src/polling_error_policy.rs#L312-L315

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/gax/src/polling_error_policy.rs#L333-L336

If a caller configures a policy with `Duration::MAX` (e.g. to disable time limits), `state.start + maximum_duration` panics immediately on the first request attempt. `checked_add` should be used instead.

Start in src/gax/src/retry_policy.rs and src/gax/src/polling_error_policy.rs at the linked deadline calculations. Read how maximum_duration is configured and how retry and polling deadlines are evaluated, then reproduce the Duration::MAX case. Done means the first request attempt no longer panics when the configured duration overflows, with the relevant gax tests passing.

Written by the indexing model from the issue text.