gh-154719: Preserve trailing whitespace in t-string interpolation expressions - #154762

1. Interpolation.expression did not match its documented contract

Doc/library/string.templatelib.rst states that, for interpolations created by t-string literals, expression is

the expression text found inside the curly brackets ({ & }), including
any whitespace, excluding the curly brackets themselves, and ending before
the first !, :, or = if any is present.

Whitespace after { was preserved, but whitespace before }, !, : or a debug = was removed:

>>> t"{ x }".interpolations[0].expression
' x'      # expected ' x '
>>> t"{  x  }".interpolations[0].expression
'  x'     # expected '  x  '
>>> t"""{
...   x
... }""".interpolations[0].expression
'\n  x'   # expected '\n  x\n'

The asymmetry is not documented anywhere: leading whitespace was kept, trailing whitespace was dropped.

2. ast.unparse() could emit code that no longer parses

When the interpolation ended with an explicit line continuation, stripping the trailing whitespace also removed the newline that terminated it, leaving a dangling backslash in Interpolation.str:

>>> src = 't"""{x \\\n}"""'
>>> ast.parse(src).body[0].value.values[0].str
'x \\'                     # the line continuation lost its newline
>>> ast.unparse(ast.parse(src))
"t'{x \\}'"
>>> ast.parse(ast.unparse(ast.parse(src)))
SyntaxError: unexpected character after line continuation character

Root cause

_strip_interpolation_expr() in Parser/action_helpers.c was applied to every interpolation, not just debug ones. It walked backwards over the lexer metadata removing every trailing whitespace character and =, which is only correct for
the debug (=) form, where the trailing = and the text after it belong to the debug text rather than to the expression.

Solution

• Regular interpolations now use the lexer metadata verbatim.
• Debug interpolations keep the text that precedes the debug = and discard only the = itself plus the whitespace and explicit line continuations that follow it. Per the documentation the expression ends before the =, so t"""{value =\<newline>}""" yields 'value ', and the \<newline> stays in the debug text.
• If the debug marker turns out to be absent, the metadata is returned unchanged instead of silently dropping source text.

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