test PD follower fallback on stale regions

[okJiang]

What changed

This PR adds a RegionCache unit test for active follower fallback when PD follower responses are stale or incomplete.

The test covers three region APIs:

• GetRegion: the first follower-enabled request returns a stale pre-split region. The test verifies client-go rejects it as stale, retries without follower/router options, and gets the fresh leader result.
• ScanRegions: the first follower-enabled request returns a stale region that cannot cover the requested range. The test verifies the gap path retries without follower/router options and gets the complete leader result.
• BatchScanRegions: covers the same gap fallback behavior for batch scan.

Why

A normal successful request does not prove that active follower fallback worked. This regression test checks request options, retry counts, and final region IDs/ranges so the fallback path is explicitly exercised.

Tests

go test ./internal/locate -run '^TestRegionCache$/^TestRegionAPIFallbackFromStaleFollowerToLeader$' -count=1
go test ./internal/locate -count=1

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