#12503: feat: add DeepSeek provider suppor

Adds support for DeepSeek provider Replaces #11151 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds DeepSeek as a first-class model provider across onboarding and provider-resolution: - Adds DeepSeek provider metadata (base URL, default models, costs) and makes it discoverable via implicit provider resolution when a DeepSeek API key is present. - Extends onboarding flows to support selecting DeepSeek (`deepseek-api-key`) in both interactive auth-choice handling and non-interactive `--deepseek-api-key` flag paths. - Adds config helpers to register the DeepSeek provider + models and set DeepSeek as the default model. - Includes new vitest coverage for DeepSeek config application and non-interactive onboarding behavior. Overall it plugs into the same provider/config patterns used for Qianfan/Moonshot/etc, but there are a couple integration/test consistency issues called out in comments that should be fixed before merge. <h3>Confidence Score: 4/5</h3> - This PR is close to safe to merge, but has a couple integration/test correctness issues to address first. - Core DeepSeek provider wiring follows existing patterns and includes tests, but CLI help text is out of sync with supported auth choices, and there are correctness concerns around a non-interactive test’s env cleanup plus some newly added option fields that appear unused/unwired. - src/cli/program/register.onboard.ts, src/commands/onboard-non-interactive.deepseek.test.ts, src/commands/auth-choice.apply.ts <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->