fix(cosh): restore bash option after canceling from provider screen

[kongche-jbw]

Description

When a user enters the Custom Provider screen without making changes and
presses Escape to go back, the "Continue to Bash" option disappears from
the authorization dialog. The root cause is that cancelAuthentication
unconditionally sets showBashOptionInAuthDialog to false. The fix
restores it to its initial value (showBashOptionOnStartup) so the bash
option reappears after cancellation. The same fix is applied to
openAuthDialog.

Related Issue

fixes #385

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional change)
• Performance improvement
• CI/CD or build changes

Scope

• cosh (copilot-shell)
• sec-core (agent-sec-core)
• skill (os-skills)
• sight (agentsight)
• tokenless (tokenless)
• Multiple / Project-wide

Checklist

• I have read the Contributing Guide
• My code follows the project's code style
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly
• For cosh: Lint passes, type check passes, and tests pass
• For sec-core (Rust): cargo clippy -- -D warnings and cargo fmt --check pass
• For sec-core (Python): Ruff format and pytest pass
• For skill: Skill directory structure is valid and shell scripts pass syntax check
• For sight: cargo clippy -- -D warnings and cargo fmt --check pass
• For tokenless: cargo clippy -- -D warnings and cargo fmt --check pass
• Lock files are up to date (package-lock.json / Cargo.lock)

Testing

1. Start cosh without auth configured — verify "Continue to Bash" visible
2. Select "Custom Provider", press Escape without entering credentials
3. Verify "Continue to Bash" option is still visible
4. make format — passes
5. npm run lint:ci — passes
6. make test — 396 files, 7278 tests all pass

Additional Notes

Changes: useAuth.ts (fix) + useAuth.test.ts (new tests)

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[samchu-zsl]

LGTM, thx!