[Refactoring]: Migrate string position and length parameters

## Parent

- #28893

## Dependency

Depends on the shared contract delivered by the binding-layer foundation subtask.

## Scope

Migrate ordinary string position/count/length consumers to the approved integer-parameter contract:

- LEFT, RIGHT, SUBSTRING/SUBSTR/MID
- LPAD, RPAD, INSERT, LOCATE
- REPEAT, SPACE, ELT

SPLIT_PART and REGEXP optional selectors are tracked in #28979 because their existing execution signatures use narrower UINT32/INT8 contracts and require explicit range-preserving executor work rather than metadata-only migration.

Retain historical execution identities for compatibility and remove them only from new overload selection.

## Acceptance criteria

- All listed integer-only parameters use shared binding coercion and canonical signed ranges.
- Fractional constants, columns, prepared values, selectors, NULL, and overflow have focused coverage.
- Existing persisted identities remain executable.
- No new coercion semantics, protocol identity, or global binder special case is introduced. Any such need must return to design review rather than expanding this migration PR.