bug(services/hf): avoid extra HEAD probe for bucket reads

### Describe the bug

Users reported that reads from Hugging Face bucket through OpenDAL are slow and can hit rate limits.

For bucket repos, OpenDAL currently still calls `HfCore::maybe_xet_file()` before creating the reader. This sends a HEAD request to the resolve URL on every read to discover `X-Xet-Hash` and size metadata, even though buckets are expected to use XET.

Relevant code:

- `core/services/hf/src/reader.rs`: `HfReader::try_new()` calls `core.maybe_xet_file(path)` before reading.
- `core/services/hf/src/core.rs`: `maybe_xet_file()` issues a HEAD request and parses `X-Xet-Hash`.
- `core/services/hf/src/backend.rs`: bucket `stat()` uses the same probe.

### Steps to Reproduce

Read many objects or ranges from an HF bucket-backed OpenDAL operator.

The issue is easier to observe with workloads that repeatedly call read or range-read, such as parquet/object_store-style readers.

### Expected Behavior

HF bucket reads should avoid an extra per-read HEAD probe when the required XET metadata can be resolved through a bucket-native path, cached metadata, list/stat metadata, or another lower-request-count mechanism.

A normal read path should not double the number of HF-facing requests before data transfer starts.

### Additional Context

This is separate from #7577, which tracks HF dataset behavior-test timeouts on CI. This issue is specifically about HF bucket read request amplification and rate limiting.

Related broader API work: #5872 tracks returning metadata from read operations, which can help integrations avoid extra stat calls.

Start in core/services/hf/src/reader.rs at HfReader::try_new(), then trace HfCore::maybe_xet_file() in core/services/hf/src/core.rs and the bucket stat path in core/services/hf/src/backend.rs. Run repeated bucket read and range-read workloads to measure HF-facing requests. Done means bucket reads avoid the extra per-read HEAD probe while still obtaining the required XET metadata and transferring data correctly.

Written by the indexing model from the issue text.