allow innodb_lock_wait_timeout as system variable (#16574)

[mhamza15]

Description

This is a backport of vitessio#16574

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[Copilot]

Pull Request Overview

This PR adds support for the innodb_lock_wait_timeout system variable by moving it from the read-only variables list to the settable variables list, enabling it to be modified at the session level.

• Moves innodb_lock_wait_timeout from read-only to settable system variables
• Updates AST rewriting to exclude global scope variables from rewriting
• Adds comprehensive test coverage for both session and global variable behavior

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/vt/sysvars/sysvars.go	Moves innodb_lock_wait_timeout from read-only to settable variables list
go/vt/sqlparser/ast_rewriting.go	Updates variable rewriting logic to exclude global scope variables
go/vt/vtgate/executor_set_test.go	Adds unit tests for setting innodb_lock_wait_timeout
go/vt/vtgate/executor_select_test.go	Adds comprehensive test for global vs session variable behavior
go/test/endtoend/vtgate/reservedconn/sysvar_test.go	Adds end-to-end test for innodb_lock_wait_timeout functionality

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