Add Python callback provider for custom unit resolution

[aytey]

Overview

This PR introduces a callback-based unit provider that lets Python applications implement custom unit resolution logic without requiring GPR project files or predefined file lists.

The existing providers (for_project and auto) work well for standard Ada projects, but many applications need dynamic resolution based on runtime configuration, database lookups, virtual file systems, or non-standard naming conventions.

API

def my_resolver(name: str, kind: str) -> str | None:
    """
    Args:
        name: Unit name (e.g., "ada.text_io")
        kind: "spec" or "body"

    Returns:
        Path to source file, or None if not found
    """
    suffix = ".ads" if kind == "spec" else ".adb"
    return f"src/{name.replace('.', '-')}{suffix}"

provider = lal.UnitProvider.from_callback(my_resolver)
ctx = lal.AnalysisContext(unit_provider=provider)

Implementation

The implementation spans three layers:

• Ada (libadalang-callback_provider.adb/ads) — implements Unit_Provider_Interface, manages callback invocation and memory ownership for returned filenames
• C FFI (libadalang-implementation-c-extensions.adb/ads) — provides C-compatible callback types, handles charset conversion, documents memory contracts
• Python (extensions/python_api/unit_providers/methods) — wraps C callbacks, handles malloc/free for string returns, retains callback references to prevent GC

Memory ownership: Python allocates filename strings with malloc(), Ada calls free() after copying. NULL returns indicate "unit not found".

Exceptions in callbacks are logged via _log_uncaught_error and the callback returns NULL, allowing analysis to continue.

Limitations

• Assumes one compilation unit per file (PLE_Root_Index := 1)
• The documentation example using a temporary context is limited to syntactic analysis

Testing

Tests cover basic resolution, None returns, exception handling, charset support (UTF-8 and default), multiple invocations, memory stress (110+ invocations), and Unicode unit names. All pass.

Documentation

The user manual now documents UnitProvider.from_callback() with a practical example, memory ownership contracts for C API users, and the limitations above.

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[aytey]

As a note, I tested this with the 26.0 branches -- I didn't actually test with master (because I build the "stable" branch); I just rebased my work from 26.0 onto master.