firestore: field path with a quoted name ending in a backslash is misparsed #18771

Package: google-cloud-firestore (google/cloud/firestore_v1/field_path.py), current main.

What happens

FieldPath.to_api_repr() escapes a trailing backslash correctly, but the parser cannot read the result back when another quoted segment follows it. FieldPath.from_api_repr() raises ValueError, and FieldPath.from_string() catches that error and falls back to a plain split on ., so it silently returns parts that still contain the backticks and escape characters.

Because update() and set(..., merge=[...]) go through FieldPath.from_string() / parse_field_path(), the write goes to the wrong field.

Minimal repro

from google.cloud.firestore_v1.field_path import FieldPath
from google.cloud.firestore_v1 import _helpers

s = FieldPath("dir\\", "file name").to_api_repr()
print(s)                                  # `dir\\`.`file name`
print(FieldPath.from_string(s).parts)     # ('`dir\\\\`', '`file name`')
FieldPath.from_api_repr(s)                # ValueError: Invalid path

pbs = _helpers.pbs_for_update("projects/p/databases/d/documents/c/doc", {s: 1}, None)
print(list(pbs[0].update.fields))         # ['`dir\\\\`']

What should happen

The string produced by to_api_repr() should parse back to the same parts, ('dir\\', 'file name'), and the update should target dir\ > file name.

Root cause

The QUOTED token pattern is r"`(?:\\`|[^`])*?`". In `dir\\`.`file name` the lazy match takes the second backslash together with the closing backtick as an escaped backtick (\`), so the token runs on to the next backtick and swallows `.`. What remains (file name`) does not tokenize, so parsing fails. The pattern needs to treat a backslash as an escape for the next character, not only for a backtick, and should not let a bare backslash match on its own.