Error finding stub app on macOS when formal name includes a .

[freakboy3742]

Describe the bug

If the formal name of an app contains a period, Briefcase will fail to build the app, as the unbuilt_executable_path will contain the "suffix" suggested by the period.

Steps to reproduce

1. briefcase new --no-input -Q "formal_name=Hello 2.0"
2. briefcase run
3. See error:

Traceback (most recent call last):
  File "/Users/rkm/beeware/briefcase/venv3.13/bin/briefcase", line 8, in <module>
    sys.exit(main())
             ~~~~^^
  File "/Users/rkm/beeware/briefcase/src/briefcase/__main__.py", line 27, in main
    command(**options)
    ~~~~~~~^^^^^^^^^^^
  File "/Users/rkm/beeware/briefcase/src/briefcase/commands/run.py", line 309, in __call__
    state = self.build_command(
        app,
    ...<7 lines>...
        **options,
    )
  File "/Users/rkm/beeware/briefcase/src/briefcase/commands/build.py", line 127, in __call__
    state = self._build_app(
        app,
    ...<7 lines>...
        **options,
    )
  File "/Users/rkm/beeware/briefcase/src/briefcase/commands/build.py", line 52, in _build_app
    state = self.create_command(app, test_mode=test_mode, **options)
  File "/Users/rkm/beeware/briefcase/src/briefcase/commands/create.py", line 984, in __call__
    state = self.create_app(app, **options)
  File "/Users/rkm/beeware/briefcase/src/briefcase/commands/create.py", line 938, in create_app
    self.install_stub_binary(app=app)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/rkm/beeware/briefcase/src/briefcase/commands/create.py", line 433, in install_stub_binary
    self.tools.os.chmod(unbuilt_executable_path, 0o755)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/Users/rkm/beeware/briefcase/local/tmp/hello20/build/hello20/macos/app/Hello 2.0.app/Contents/MacOS/Stub.0'
(

Expected behavior

App should build and run successfully.

Screenshots

No response

Environment

• Operating System: macOS
• Python version: any (tested on 3.13, 3.12)
• Software versions:
  • Briefcase: 0.3.21+

Logs

Additional context

Originally reported by @rbrecheisen in this comment on #1729.

[freakboy3742]

This is caused by the interaction of Windows and macOS executable handling. The evaluation of unbuilt_executable_path splits the suffix off the binary path so that it can create Stub.exe; but if you're on macOS, the same logic will strip .0 off the name Hello 2.0 as the "suffix".

We need to make the suffix handling platform specific, so that the .suffix handling is only performed on Windows.