[copybara-service]

[langfun] Make Template.raw_str total over arbitrary text

Template.raw_str(text) wrapped text in {% raw %} ... {% endraw %} to
preserve it verbatim. A raw block ends at the first endraw tag, so text that
itself contained one closed the block early: the remainder was parsed as Jinja,
which raised Bad template string on a {% ... %} sequence and registered an
undeclared variable on a {{ ... }} one. The function whose job is to quote
arbitrary text was not total over it, and neither failure was fixable by the
caller, since the escaping it would have to redo is the escaping that failed.

• Each literal endraw tag is now emitted from outside the raw block -- its
  {% as a Jinja string expression, the remainder from a reopened raw block --
  so the template renders back to exactly the input and declares no variables.
  _ENDRAW_TAG matches every spelling that ends a block, including the
  whitespace-control forms, mirroring the TOKEN_RAW_BEGIN rule of
  jinja2.lexer. Everything else, {% raw %} itself included, is already
  literal inside a raw block, which does not nest.

• Template.from_raw_str(text, **kwargs) forwards template attributes and
  variables instead of discarding them. It accepted only text, so a caller
  that needed any other attribute had to re-implement the escaping rather than
  reuse it. clean stays forced off, because reindenting the text would
  contradict preserving it as original.