What problem does this PR solve?

Issue Number: close #10950

Problem Summary:

When diagnosing TiFlash storage and query issues in production, existing logs and execution summaries lack enough context to distinguish scan types and understand slow storage operations:

• TableScan execution summary JSON does not indicate whether the scan is a regular TableScan or PartitionTableScan.
• Split-based ingest logs are mostly at DEBUG level and do not include elapsed time, making slow ingest hard to spot.
• Segment split/merge-delta finish logs do not include the trigger reason.
• Segment info strings miss stable column count and on-disk DMFile size.
• FileCache download success logs do not distinguish foreground vs background downloads.
• gtest_kvstore_fast_add_peer is flaky in CI and FAP is not planned for short-term support.

What is changed and how it works?

Add observability improvements for TableScan statistics and storage layer logging.

- Expose `is_partition_table_scan` in TableScan execution summary JSON.
- Add elapsed-time logging for split ingest; promote slow paths (>10s) from DEBUG to INFO.
- Log split/merge-delta reason in segment operation finish logs.
- Enrich Segment::info() with stable column count and DMFile on-disk bytes.
- Log FileCache download type (Foreground/Background) on successful download.
- Disable flaky gtest_kvstore_fast_add_peer test.

TableScan statistics

• Cache is_partition_table_scan in TableScanStatistics constructor via executor->has_partition_table_scan().
• Output "is_partition_table_scan": true/false in appendExtraJson, so execution summary / slow query analysis can tell partition scans apart.

DeltaMerge storage logging

• Introduce SplitIngestLogContext in ingestDTFilesUsingSplit:
  • Track elapsed time across split ingest phases.
  • Use LOG_IMPL to emit begin/finish logs at INFO when elapsed > 10s, otherwise DEBUG.
  • Add elapsed_seconds to per-file ingest attempt logs (INFO).
• Log reason in segmentSplit and segmentMergeDelta finish logs via magic_enum::enum_name(reason).
• Extend Segment::info() with stable_cols and dmf_disk_bytes.
• Add DMFile::getNumColumns() and StableValueSpace::getDMFilesNumColumns().

S3 FileCache

• Add FileCache::DownloadType (Foreground / Background).
• Include download type in downloadImpl success log for easier disagg read path analysis.

Tests

• Wrap gtest_kvstore_fast_add_peer.cpp with #if 0 and document why (flaky CI, FAP not planned for short-term support).

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

Summary by CodeRabbit

• New Features
  • Enhanced table-scan status details with partition-scan indicator and added stable scan summary fields.
  • Added column-count reporting to segment/stable summaries.
  • S3 file-download logging now distinguishes foreground vs background transfers.
• Bug Fixes
  • Improved split-ingest and completion logging with elapsed time and “reason” context for easier troubleshooting.
  • Disabled a flaky KV store test from building/running.
• Refactor
  • Clarified the split-vs-column-file ingest branching for improved readability.