sql: prevent unsafe comparison constant unification

[shivamshaw23]

Summary

Fixes #172978.

UnifyComparison could replace a mixed-type comparison with one against a converted constant even when the comparison's implicit cast collapses distinct column values. This caused INT8 values above 2^53 to be missed when compared to a FLOAT8 constant. It also performed stable casts without consulting FoldingControl, allowing session-time-zone-dependent TIMESTAMP to TIMESTAMPTZ conversions to be baked into prepared plans.

This change only permits the rewrite when the variable-to-original conversion is injective, and consults cast volatility through FoldingControl before evaluating either cast. It adds logic tests for scalar and ANY comparisons and for prepared statements across a time-zone change.

Validation

• git diff --check
• Full Bazel logic-test generation was not run locally: WSL fails to start because its Docker disk path is missing.
• Direct go test ./pkg/sql/opt/norm is not supported in this checkout without Bazel-generated protobuf packages.

[blathers-crl]

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• @yuzefovich (author of sql/opt: constant type-unification in comparisons yields incorrect results (large INT vs FLOAT; TZ-dependent casts in prepared plans) #172978)

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[shivamshaw23]

Review focus: please validate the intentionally conservative isInjectiveConversion allowlist and whether any additional type pairs are provably injective. The key invariant is that rewriting variable <op> constant must preserve the comparison overload's implicit cast for every possible variable value, not merely round-trip the selected constant.

The regression covers both paths from #172978 (scalar/ANY INT8-vs-FLOAT8 and prepared TIMESTAMP/TIMESTAMPTZ after a time-zone change). Local full test execution is blocked by the machine's unavailable WSL/Docker disk, as described in the PR body.