Fix bs_dependency_defer() cache collisions for factory-built closures

[AleKoure]

Closes #1330

Summary

bs_dependency_defer() memoises dependency functions using a shared cache. The
memoise key includes the function's formals and body, but not its enclosing
environment.

As a result, closures created by the same factory can collide when they differ
only in captured values. After the first dependency is cached, subsequent
dependencies may incorrectly receive the first dependency's compiled output.

This PR adds an optional cache_key argument:

bs_dependency_defer(func, memoise = TRUE, cache_key = NULL)

When supplied, cache_key is folded into the memoise key, allowing callers to
distinguish factory-generated dependency functions. Existing behavior is
unchanged when cache_key = NULL.

When memoise = FALSE, cache_key has no effect and a warning is emitted.

Reproducible example

library(bslib)

mk_dep <- function(name, css) {
  bs_dependency_defer(
    function(theme) {
      if (!is_bs_theme(theme)) {
        theme <- bs_theme(version = 5)
      }

      bs_dependency(
        input = css,
        theme = theme,
        name = paste0("test-", name),
        version = "0.0.0"
      )
    },
    cache_key = name
  )
}

mk_dep("red", ".x { color: red }")()$name
#> [1] "test-red"

mk_dep("blue", ".x { color: blue }")()$name
#> [1] "test-blue"

Without distinct cache keys, both factory-generated closures have identical
formals and bodies and can resolve to the same cached dependency.

Documentation

The bs_dependency_defer() documentation now describes when cache_key is
needed. The custom-components vignette also notes that helper-generated
dependency functions should use a stable, distinct cache key.

Testing

Added unit tests covering:

• distinct factory-generated dependencies with cache_key;
• the existing collision without cache_key;
• memoise = FALSE;
• the warning when cache_key is combined with memoise = FALSE;
• unchanged caching behavior for functions that are already distinct.

devtools::check() completes with 0 errors, 0 warnings, and 0 notes.

Checklist

• Relevant issue exists and the contribution was welcomed by a maintainer.
• Unit tests were added under tests/testthat.
• Documentation was updated.
• devtools::check() passes.
• Code follows the project style.
• NEWS.md includes an entry for the change.

[CLAassistant]

All committers have signed the CLA.