flash: handle TiCI estimate count RPC

[wshwsh12]

What problem does this PR solve?

Issue Number: None

Problem Summary:

TiDB needs TiFlash to expose TiCI-side estimate count so the optimizer can use sampled TiCI fulltext cardinality instead of relying only on local planner estimates.

What is changed and how it works?

flash: handle GetEstimateTiCICount RPC for TiCI fulltext stats

• Add FlashService handling for GetEstimateTiCICount.
• Build TiCI estimate input from the request expression and shard/range list.
• Return TiCI estimate count to TiDB and ignore missing shards during estimate.
• Update kvproto to the branch containing the estimate count RPC definitions.

Check List

Tests

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

Manual test:

• TiUP playground tag: tici-estimate-skew-20260426-2240.
• Query: EXPLAIN SELECT COUNT(*) FROM estimate_skew WHERE MATCH(content) AGAINST ('heavyhit' IN BOOLEAN MODE);.
• Result: TiDB used TiCI estimate 4321; exact match count was 4000; total table row count was 9400.
• Logs confirmed TiFlash GetEstimateTiCICount done, est_count=4321 and TiCI EstimateCountResult { estimated_total_count: 4321, ... }.

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

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