fix: align GetDefaultDetectionModel with main agent default (claude-sonnet-4.6) (#23160)

* fix: align GetDefaultDetectionModel with main agent default (claude-sonnet-4.6)

Remove the hardcoded gpt-5.1-codex-mini default from GetDefaultDetectionModel() so
detection uses the same behavior as the main agent: no explicit model → Copilot CLI
picks its native default (currently claude-sonnet-4.6).

- Remove DefaultCopilotDetectionModel constant
- Remove GetDefaultDetectionModel() override from CopilotEngine (falls through to BaseEngine which returns "")
- Update all related tests

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/a48363b4-9ebc-49bf-aaf5-3a17dbe13686

Co-authored-by: davidslater <12449447+davidslater@users.noreply.github.com>

* Add changeset [skip-ci]

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: davidslater <12449447+davidslater@users.noreply.github.com>
Co-authored-by: David Slater <davidslater@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: 3 people