Fix genai --compile fallback to use relative mirrored ONNX path

[DingmaomaoBJTU]

Why

Running winml perf -m <bundle> --runtime winml-genai --device npu --compile failed to load the bundle whenever a pipeline stage's offline EPContext compilation crashed and fell back to JIT. This was hit in practice benchmarking a qwen3 export on a Snapdragon X Elite NPU: the context stage's offline compile aborted, and the fallback then produced a malformed path and errored out with "Failed to load genai bundle".

Root cause

ort-genai resolves each pipeline stage's filename relative to the directory it loads genai_config.json from (the _compiled dir) via a naive join, and does not accept absolute Windows paths. In _prepare_compiled_bundle, the compile-failure fallback (and the already-precompiled-stage branch) patched the stage filename to an absolute source path and skipped mirroring that ONNX into the compiled dir. ort-genai then joined the compiled dir onto the absolute path, yielding a broken _compiled\C:\...\ctx.onnx that did not exist.

Fix

Reference the original ONNX by its bundle-relative filename and let _mirror_non_onnx_files symlink it (plus its external-weights .data sidecar) into the compiled dir, exactly like the fresh-compile path already does. The mirror skip-set now contains only the stages that were actually replaced by a {stage}_ctx.onnx artifact, so fallback / already-compiled stages get mirrored in correctly.

Notes for reviewers

• This only fixes the fallback behavior. The underlying QNN offline-compile crash (native 0xC0000005 in the EPContext compiler subprocess) is a separate pre-existing issue and is not addressed here; the fix ensures the JIT fallback after that crash loads correctly rather than dying on a bad path.
• Added two regression tests covering the compile-failure fallback and the already-precompiled-stage case, asserting the stage is referenced by a relative filename and that the ONNX (+ .data sidecar) is physically present in the compiled dir.
• Verified end-to-end on the NPU: --compile now completes (context stage falls back to JIT, bundle loads, benchmark runs) with the malformed-path error gone.