Fix panic in shouldRetry type assertion and remove deprecated rand.Seed

[Nepomuk5665]

Summary

• Fix potential panic in shouldRetry function by using comma-ok idiom for type assertion
• Remove deprecated rand.Seed call that was re-seeding on every backoff attempt
• Add test coverage for shouldRetry with various error types

Bug Description

Type Assertion Panic

The shouldRetry function in retries/retries.go used a direct type assertion:

e := err.(*Err)  // This panics if err is not *Err type

While this function is internal and typically receives *Err values from Wait and Poll functions, a direct type assertion is unsafe. If any code path passes a non-*Err error, the program will panic with a runtime error.

The fix uses the comma-ok idiom to safely handle any error type:

e, ok := err.(*Err)
if !ok || e == nil {
    return false
}

Deprecated rand.Seed

The backoff function called rand.Seed(time.Now().UnixNano()) on every invocation. This has two issues:

1. Deprecated since Go 1.20: The global random number generator is now automatically seeded
2. Wasteful: Re-seeding on every retry attempt is unnecessary and has performance implications

Testing

• All existing tests pass
• Added new test TestShouldRetryWithNonErrType that verifies the fix handles various error types without panicking

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-go

Inputs:

• PR number: 1438
• Commit SHA: 0cefa2609c108943d86c68300e2cce5ca343f817

Checks will be approved automatically on success.