sql/inspect: add hash-based precheck to index consistency validation

[spilchen]

Before performing the expensive full join to compare primary and secondary indexes during the INSPECT index consistency check, this change adds a fast, low-cost precheck step.

We now compute and compare row counts and hash values from both indexes. If the counts and hashes match, we can confidently skip the full check. This optimization should improves performance for healthy indexes.

Closes: #150927
Epic: CRDB-55075
Release note: None

[cockroach-teamcity]

This change is 

[spilchen]

Here are the results of the performance tests when I ran this in roachtest:

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1. Admission Control Test (25M rows)

Fast version:

• Calibration INSPECT (no workload): 3m58.8s (3,272 rows/s/cpu)
• INSPECT under load (AC enabled): 3m21.7s (3,874 rows/s/cpu)
• INSPECT under load (AC disabled): 42.9s (18,223 rows/s/cpu)

Slow version:

• Calibration INSPECT (no workload): 11m6.6s (1,172 rows/s/cpu)
• INSPECT under load (AC enabled): 24m55.5s (522 rows/s/cpu)
• INSPECT under load (AC disabled): 2m24.8s (5,396 rows/s/cpu)

Performance improvement (fast vs slow):

• Calibration: 2.8x faster (11m6s → 3m59s)
• Under load with AC: 7.4x faster (24m55s → 3m22s)
• Under load without AC: 3.4x faster (2m25s → 43s)

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2. Throughput Test - 500M rows, 1 index

Fast version:

• INSPECT execution: 32m0.4s

Slow version:

• INSPECT execution: 58m53.9s

Performance improvement:

• 1.8x faster (58m54s → 32m0s)

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3. Throughput Test - 1B rows, 2 indexes

Fast version:

• INSPECT with 1 check: 1h53m0.5s
• INSPECT with 2 checks: 2h29m45.4s

Slow version:

• INSPECT with 1 check: 2h25m15.8s
• INSPECT with 2 checks: 3h51m30.7s

Performance improvement:

• 1 check: 1.3x faster (2h25m16s → 1h53m1s)
• 2 checks: 1.6x faster (3h51m31s → 2h29m45s)