fix: str_replace_all() error with named input and function replacement

[LeonidasZhak]

Fixes #595

Problem

str_replace_all() errors when all three conditions are met:

1. string is a named character vector
2. replacement is a function
3. The pattern matches multiple times in at least one element

str_replace_all(c(name = "string string"), "string", function(x) rep("strong", length(x)))
#> Error in vctrs::list_unchop():
#> ! Can't merge the outer name 'name' with a vector of length > 1.

Root Cause

str_transform_all() passes the named list from str_sub_all() directly to vctrs::list_unchop(), which can't propagate outer names when inner vectors have length > 1. This was introduced when the function switched from unlist() to list_unchop().

Fix

Three minimal changes in str_transform_all() (R/replace.R):

1. Save names(string) before processing
2. Use unname(old) in list_unchop()
3. Restore names after replacement

Tests

• Added test for the exact bug scenario (named input + function replacement + multiple matches)
• Added test with multiple named elements and function replacement
• All existing tests pass: FAIL: 0 | WARN: 0 | SKIP: 39 | PASS: all remaining

[LeonidasZhak]

Withdrawing this because checks are failing and I should not ask maintainers to review an unready automated PR. Sorry for the noise, and thank you for maintaining stringr.