Fix mutable generic at lookup

[sean0407]

Summary

glz::generic::at(key) currently delegates to mutable operator[], which inserts a missing object key. Route mutable at() through the existing checked const lookup, then return the existing value as a mutable reference.

Fixes #3000.

mutable at(key)
  before: operator[](key)   // inserts when absent
  after:  checked lookup    // errors when absent; never inserts

Evidence

• Before: An isolated C++23 reproducer confirmed that json.at("missing") inserted the key.
• After: The same isolated check confirmed that existing-key lookup still works, while missing-key lookup throws and leaves the object unchanged.
• Added a regression case for both existing and missing keys.

Merge Danger

Door: two-way

Blast Radius: low

[packit-as-a-service]

One of the tests failed for 45e9b96. @admin check logs https://download.copr.fedorainfracloud.org/results/packit/stephenberry-glaze-3006/fedora-rawhide-ppc64le/11155363-glaze/builder-live.log, packit dashboard https://dashboard.packit.dev/jobs/copr/4087103 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/11155363/