feat: add FieldValueIter support for Hsetex command

[kotahorii]

Summary

This PR adds FieldValueIter method support to the Hsetex command, providing API consistency with other hash commands like Hset and Hmset.

Changes

• Added HsetexFieldValue.FieldValueIter method in internal/cmds/iter.go

  • Follows the same pattern as existing HsetFieldValue.FieldValueIter implementation
  • Accepts iter.Seq2[string, string] for Go 1.23 iterator support
  • Maintains zero-allocation design philosophy

• Added comprehensive test coverage in internal/cmds/iter_test.go

  • Added Hsetex test case to the existing iterator test suite
  • Verified compatibility with maps.All() function

API Usage Example

// Before: Only one-by-one field addition
client.B().Hsetex().Key("key").Ex(60).Fields().Numfields(2).
    FieldValue().FieldValue("name", "John").FieldValue("email", "[email protected]").
    Build()

// After: Bulk field addition from map or any iter.Seq2[string, string]
data := map[string]string{"name": "John", "email": "[email protected]"}
client.B().Hsetex().Key("key").Ex(60).Fields().Numfields(2).
    FieldValue().FieldValueIter(maps.All(data)).
    Build()

Testing

• ✅ All existing tests pass
• ✅ New iterator test added and passes
• ✅ Hash command tests continue to pass
• ✅ Manual verification with sample data confirms correct command generation

Backward Compatibility

This change is fully backward compatible - no existing APIs are modified, only a new method is added.

Fixes #888

🤖 Generated with Claude Code

[rueian]

Hi @kotahorii, thanks for your PR.
Your commit has been merged into #889.