feat: allow disabling model warning browser prompts

[KXHXK]

Summary

• add --open-model-warnings / --no-open-model-warnings so browser prompts can be disabled independently
• keep model metadata and cost warnings visible in the terminal
• document the CLI, environment-variable, and YAML configuration forms
• add regression coverage proving the warning remains visible while offer_url is skipped

Motivation

Headless automation commonly uses --yes, which also accepts the model-warning documentation prompt and opens a browser on every new process. The existing --no-show-model-warnings option prevents the popup but also hides useful terminal diagnostics. This change separates those two behaviors while preserving the current default.

Validation

• Python AST parsing passed for aider/args.py, aider/main.py, and tests/basic/test_main.py
• branch comparison is one commit ahead of main with only the eight intended files changed
• focused regression test added; the full pytest run is left to CI because the local GitHub source checkout repeatedly timed out

Fixes #5547

[CLAassistant]

All committers have signed the CLA.