Add arbitrary mux server command argument parsing

[ibetitsmike]

Summary

• add a new additional_arguments module variable to pass extra arguments to mux server
• parse additional_arguments in run.sh with quoted-group support so values like paths with spaces are preserved
• keep existing add-project behavior while allowing additional arbitrary flags
• add Terraform and Bun tests covering additional_arguments behavior
• document the new option in the module README and bump example version references to 1.2.0

Why

The module previously only supported the add-project flag. This change lets users pass additional mux server arguments without waiting for new module variables.

Validation

• shellcheck --severity=warning --format=gcc registry/coder/modules/mux/run.sh
• terraform -chdir=registry/coder/modules/mux test -verbose
• bun test registry/coder/modules/mux/main.test.ts

Breaking changes

None.

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Generated with Mux (exec agent) using GPT-5.

[ibetitsmike]

Addressed review feedback: renamed the new mux argument field to "additional_arguments" (consistent naming with other modules), and updated script wiring, tests, and README examples.

[matifali]

LGTM