feat: add AsyncThrowingStream piping and completion utilities

[pepicrft]

TL;DR

Added stream piping functionality and completion handling to AsyncThrowingStream

What changed?

• Added pipedStream() method to pipe standard output and error through the process' standard output and error
• Added awaitCompletion() method to wait for stream completion
• Migrated CI to use macos-15 runner and simplified Swift setup
• Removed explicit Xcode version management
• Updated tests to use the new Testing framework

How to test?

1. Run a command with pipedStream() to verify output is correctly piped:

try await CommandRunner()
    .run(arguments: ["echo", "foo"])
    .pipedStream()
    .awaitCompletion()

2. Verify the output appears in the standard output
3. Run existing tests to ensure concatenatedString functionality remains intact

Why make this change?

To provide better control over command output handling by allowing direct piping to standard streams and providing a way to wait for stream completion without needing to manually consume the stream.

[pepicrft]

• feat: add AsyncThrowingStream piping and completion utilities #156 👈 (View in Graphite)
• main

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