region_cache: return nil reason from GetTiFlashRPCContext (#1959)

[ti-chi-bot]

This is an automated cherry-pick of #1959

Return a reason when GetTiFlashRPCContext returns nil to help callers know why the TiFlash RPC context is unavailable.

Summary by CodeRabbit

• New Features

  • Exposed detailed TiFlash RPC-context unavailability reasons and structured details for clearer diagnostics.

• Improvements

  • More explicit handling and reporting when TiFlash contexts are unavailable (including store lists and specific causes).
  • Improved TiFlash store selection with better load-balancing and label-based filtering for more reliable routing.

[ti-chi-bot]

@gengliqi This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

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