feat(nimble): Plumb FileIoContext through TabletReader direct preadv calls

[mkarrmann]

Summary:
TabletReader performs file IO outside the normal BufferedInput read path: postscript/footer bootstrap reads, coalesced metadata loads through MetadataInput, index metadata/data reads, and stripe data loads. Those reads can reach ReadFile implementations such as WSReadFile, which record filesystem runtime stats from FileIoContext::ioStats. When TabletReader and its helper readers use the default FileIoContext{}, those filesystem stats are dropped even though the original BufferedInput has the operator's context.

This change carries the BufferedInput's FileIoContext through the current TabletReader architecture:

• TabletReader::configureOptions(options, bufferedInput) extracts bufferedInput->getInputStream()->fileIoContext() when a BufferedInput is supplied.
• TabletReader::Options and TabletReader store that FileIoContext.
• Footer/postscript rawRead(...) calls pass the context to ReadFile::pread(...).
• MetadataInput stores the context and forwards it to coalesced ReadFile::preadv(...) calls.
• IndexLookup::Options carries the same context, and index metadata/data input construction uses it for direct metadata reads and file read options.
• Stripe-data loads continue to pass the same context to direct file_->preadv(...) calls.
• The selective Nimble reader path (ReaderBase::create) now passes its BufferedInput into configureOptions so the context is wired up automatically.
• The TrackingReadFile test helper now increments trackingReadFile.pread/trackingReadFile.preadv counters on the supplied FileIoContext::ioStats, enabling the new tests to assert that the context reaches the underlying ReadFile.

Depends on the prior commit (feat(dwio): Add public fileIoContext() accessor to ReadFileInputStream) for exposing the FileIoContext stored inside ReadFileInputStream.

Compatibility:

• Source-compatible: all new parameters default to FileIoContext{}/nullptr where existing callers do not have a BufferedInput.
• Behavior-compatible for standard ReadFile implementations that ignore FileIoContext::ioStats.
• Implementations that consume FileIoContext::ioStats, such as WSReadFile, now see the caller's filesystem stats context for TabletReader metadata and data reads.

Differential Revision: D103114180

[meta-codesync]

@mkarrmann has exported this pull request. If you are a Meta employee, you can view the originating Diff in D103114180.