tests: add whitespace tests for vertical tab behavior

* tests: add whitespace tests for vertical tab behavior

Add two small tests to highlight how vertical tab is handled differently.

- vertical_tab_lexer.rs checks that the lexer treats vertical tab as whitespace
- ascii_whitespace_excludes_vertical_tab.rs shows that split_ascii_whitespace does not split on it

This helps document the difference between the Rust parser (which accepts vertical tab)
and the standard library’s ASCII whitespace handling.

See: rust-lang/rust-project-goals#53
* tests: add ignore-tidy-tab directive to whitespace tests
* tests: expand vertical tab lexer test to cover all Pattern_White_Space chars
* tests: add whitespace/ README entry explaining lexer vs stdlib mismatch
* Update ascii_whitespace_excludes_vertical_tab.rs
* Update ascii_whitespace_excludes_vertical_tab.rs

make sure tabs and spaces are well checked
* Update ascii_whitespace_excludes_vertical_tab.rs
* fix tidy: add whitespace README entry
* Update README.md with missing full stop
* Update ascii_whitespace_excludes_vertical_tab.rs
* fix tidy: use full path format for whitespace README entry
* fix tidy: README order, trailing newlines in whitespace tests
* fix: add run-pass directive and restore embedded whitespace bytes
* fix tidy: remove duplicate whitespace README entry
* 

Add failing UI test for invalid whitespace (zero width space)

This adds a //@ check-fail test to ensure that disallowed whitespace
characters like ZERO WIDTH SPACE are rejected by the Rust lexer.
* git add tests/ui/whitespace/invalid_whitespace.rs
git commit -m "Fix tidy: add trailing newline"
git push
* Fix tidy: add trailing newline
* Update invalid_whitespace.rs
* Update invalid_whitespace.rs
* Clean up whitespace in invalid_whitespace.rs

Remove unnecessary blank lines in invalid_whitespace.rs
* Update invalid_whitespace.rs
* Clarify ZERO WIDTH SPACE usage in test

Update comment to clarify usage of ZERO WIDTH SPACE.
* Improve error messages for invalid whitespace

Updated error messages to clarify the issue with invisible characters.
* Modify invalid_whitespace test for clarity

Update test to check for invalid whitespace characters.
* Resolve unknown token error in invalid_whitespace.rs

Fix whitespace issue causing unknown token error.
* Remove invisible character from variable assignment

Fix invisible character issue in variable assignment.
* Improve error message for invalid whitespace

Updated error message to clarify invisible characters.
* Improve error handling for invisible characters

Updated error message for invisible characters in code.
* Document error for unknown token due to whitespace

Add error message for invalid whitespace in code
* Update error message for invalid whitespace handling
* Modify invalid_whitespace.rs for whitespace checks

Updated the test to check for invalid whitespace handling.
* Correct whitespace in variable declaration

Fix formatting issue by adding space around '=' in variable declaration.
* Update error message for invalid whitespace
* Update invalid_whitespace.stderr
* Refine error handling for invalid whitespace test

Update the error messages for invalid whitespace in the test.
* Update invalid_whitespace.rs
* Fix whitespace issues in invalid_whitespace.rs
* Update invalid_whitespace.stderr file
* Clean up whitespace in invalid_whitespace.rs

Removed unnecessary blank lines from the test file.
* Update invalid_whitespace.stderr