Add API to query executable paths for runnable targets

[Richlee96112]

This change adds a public API to retrieve the absolute path of the built binary for a specified executable target, addressing issue #286. This functionality is intended to support tools like swift run, which need to locate and launch built targets.

The new asynchronous API, generateRunnableInfo, is implemented in SWBBuildServiceSession and returns a SWBRunnableInfo struct containing the resolved executable path. Targets are resolved using targetID, providing reliable and consistent identification across the build system.

Comprehensive error handling is included to cover cases where the target is missing or not executable. Unit tests validate both successful and failure scenarios to ensure correctness.

• Sources/Protocol/SWBRunnableInfo.swift – Defines a struct that encapsulates the path to a built executable
• Sources/BuildService/Session.swift – Implements a new API that takes a targetID and returns the resolved path of the corresponding executable
• Tests/SWBBuildServiceTests/BuildServiceTests.swift – Adds unit tests for both success and error scenarios

Note: This PR addresses only the functionality requested in issue #286. Any existing Swift 6 compatibility issues in the project are unrelated to these changes and should be tracked and resolved separately.

While running tests, the following Swift 6 compatibility errors were encountered:
error: cannot find type 'sendingValue' in scope
error: cannot infer contextual base in reference to member 'init'
error: contextual closure type '() -> sending ()' expects 0 arguments...
These errors stem from existing code in the SWBUtil module that is not yet compatible with Swift 6’s new ownership model (e.g., ~Copyable, sending).

[owenv]

Thanks for the contribution! It doesn't look like this change compiles currently, so I'm converting it to a draft for now. Please feel free to re-request review once it's building successfully.