fix: preserve prepared arithmetic and exact integer semantics

[ck89119]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #25408
issue #27492

What this PR does / why we need it:

Prepared executions transport SQL user-variable values and binary-protocol string values through text-backed parameter vectors. Two related runtime-specialization gaps could therefore reuse or select an approximate domain that did not match the current execution:

• EXECUTE ... USING arithmetic could retain an earlier integer cast when a later execution supplied DECIMAL;
• text-backed BIGINT UNSIGNED and BIT(64) comparisons could pass 2^53+1 through DOUBLE, merging adjacent BIGINT values or raising a BIT range error.

This PR keeps the two ownership rules separate while validating them together:

• SQL arithmetic rebinding uses the current user variable's logical source type and includes it in the specialization-cache identity;
• comparison rebinding recognizes text operands against exact integer/BIT peers and materializes an in-range integral numeric prefix directly in the peer domain;
• fractional, out-of-range, nonnumeric, and DECIMAL string comparisons retain the existing MySQL-compatible DOUBLE fallback;
• BIT comparisons remain in the BIT domain, preserving the column-side comparison/index contract;
• specialized literals retain parameter provenance so same-category cached plans read each execution's current value.

The implementation is based on current main after #27483. No code was cherry-picked from the older broad implementation in #27466.

Testing

• complete pkg/sql/plan and pkg/frontend test packages
• complete TestIssue27088PreparedDecimalCommonType, including SQL PREPARE and COM_STMT BIGINT UNSIGNED/BIT(64) string, integer, NULL, and reuse transitions
• CGo-wrapper vet=all for pkg/sql/plan, pkg/frontend, and pkg/tests/issues
• make build
• prepare_all.sql in normal mo-tester comparison mode twice on the same final binary: 438 total / 437 success / 0 failed / 1 ignored / 0 abnormal in both runs
• package coverage runs: planner 79.1%, frontend 64.1%, with focused tests for every new specialization branch