[mojo][core] Update the codebase to Mojo v1.0.0b1

[forfudan]

This PR updates ArgMojo’s core code and tooling configuration to be compatible with Mojo v1.0.0b1.

Changes:

• Bump Pixi environment (project version + Mojo dependency) to Mojo 1.0.0b1 and refresh the lockfile.
• Migrate string-length usage from len(String) to String.byte_length() across core parsing/help code.
• Update reflection and type constraints for Mojo 1.0 (reflect[T]() API, ImplicitlyDestructible bounds, and thin function pointers for stored handlers).

[Copilot]

Pull request overview

This PR updates ArgMojo’s core code and tooling configuration to be compatible with Mojo v1.0.0b1.

Changes:

• Bump Pixi environment (project version + Mojo dependency) to Mojo 1.0.0b1 and refresh the lockfile.
• Migrate string-length usage from len(String) to String.byte_length() across core parsing/help code.
• Update reflection and type constraints for Mojo 1.0 (reflect[T]() API, ImplicitlyDestructible bounds, and thin function pointers for stored handlers).

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_help.mojo	Updates help-output parsing logic to use byte_length() for Mojo 1.0 string APIs.
src/argmojo/utils.mojo	Migrates string length checks to byte_length() and updates numeric/similarity helpers accordingly.
src/argmojo/parse_result.mojo	Switches alignment/emptiness checks to byte_length() for Mojo 1.0 compatibility.
src/argmojo/parsable.mojo	Updates trait bounds and replaces deprecated struct_field_* reflection with reflect[Self]() API.
src/argmojo/command.mojo	Updates response-file parsing length checks and changes auto-dispatch handler storage/signature to thin function pointers.
src/argmojo/argument.mojo	Updates compile-time validation to use byte_length() for Mojo 1.0 string semantics.
src/argmojo/argument_wrappers.mojo	Adds ImplicitlyDestructible bounds and switches type-name reflection to reflect[T]().name().
pixi.toml	Bumps project version and pins Mojo to ==1.0.0b1.
pixi.lock	Updates locked artifacts to Mojo 1.0.0b1 (+ related package updates).
docs/changelog.md	Adds an unreleased note documenting Mojo 1.0.0b1 compatibility changes.

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