feat: write inlined ChunkManifest entries to icechunk as native chunks

[TomNicholas]

Summary

Teaches the icechunk writer to handle ChunkManifest entries containing inlined chunk data. Without this, write_manifest_to_icechunk sends the "__inlined__" sentinel straight into icechunk's set_virtual_refs_arr, which rejects it with `error parsing virtual ref URL: "inlined"`.

New control flow (single bulk set_virtual_refs_arr call; fast path unchanged):

1. If the manifest has inlined chunks, write them first as native chunks — one store.set(chunk_key, buffer) per entry, all dispatched in parallel under a single sync(asyncio.gather(...)) bridge.
2. Rewrite those positions in the paths array from "__inlined__" to "" so they're treated as skipped-by-Rust positions on the subsequent call.
3. Cheap numpy-level (virtual_paths != "").any() check — skip the set_virtual_refs_arr call entirely for all-inlined or all-missing manifests (no .tolist() allocation, no Python→Rust round trip).

Together with #979 and #980, this completes end-to-end round-tripping of inlined chunks through both kerchunk and icechunk.

Test plan

• End-to-end test writes a ManifestArray with one inlined chunk plus one virtual chunk, commits, re-opens via xr.open_zarr, and asserts the expected values on both positions.
• All 39 existing icechunk writer tests still pass unchanged (they exercise the pure-virtual fast path).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.97%. Comparing base (08f95c3) to head (4634a5f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #981      +/-   ##
==========================================
+ Coverage   89.94%   89.97%   +0.02%     
==========================================
  Files          33       33              
  Lines        2058     2064       +6     
==========================================
+ Hits         1851     1857       +6     
  Misses        207      207              

Files with missing lines	Coverage Δ
virtualizarr/writers/icechunk.py	91.66% <100.00%> (+0.30%)	⬆️

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