gh-154719: Preserve trailing whitespace in t-string interpolation expressions

[lpyu001]

Issue

Interpolation.expression preserved whitespace after the opening {, but removed whitespace immediately before }, !, or :. This was inconsistent with the documented behavior of preserving the expression text inside the braces.

Root cause

The lexer metadata already contained the trailing whitespace, but the parser unconditionally stripped trailing whitespace from every interpolation expression while handling debug expressions.

Solution

Preserve the lexer metadata unchanged for regular interpolations. For self-documenting expressions, remove only the debug = and whitespace after it, while preserving whitespace before it. Add tests covering regular, converted, formatted, debug, comparison, and multiline expressions.

• Issue: Interpolation.expression drops trailing whitespace despite documentation saying it includes any whitespace #154719