Simplify project context/auth flow for read-only commands + persist project_id

[Parker Henderson (parkerhendo)]

Changes

1. bt switch now persists project_id in config alongside org and project name
2. Read-only commands (list/view operations) use fast authentication path when cached project_id exists, avoiding full login validation
3. Added login_read_only() and fast_login() functions to support optimized authentication for read operations
4. Experiments, prompts, and functions modules now route read-only commands through optimized auth path
5. Created shared ProjectContext type and resolve_project_context() function for consistent project resolution
6. Spinner display delay increased from 300ms to 500ms
7. Added comprehensive test coverage for new authentication modes and project context resolution

[github-actions]

Latest downloadable build artifacts for this PR commit f18dad5eb538:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/24217020036
• Download all artifacts (GitHub CLI): gh run download 24217020036 --repo braintrustdata/bt
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• ``artifacts-build-local-x86_64-unknown-linux-musl
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• ``cargo-dist-cache

[Olmo Maldonado (ibolmo)]

happy to try to review (rust is not yet my strong suit), but at a high level this pr could use some tests. i'll come back around today, but let me know if you'd like to proceed without tests Parker Henderson (@parkerhendo)

[Parker Henderson (parkerhendo)]

happy to try to review (rust is not yet my strong suit), but at a high level this pr could use some tests. i'll come back around today, but let me know if you'd like to proceed without tests Parker Henderson (Parker Henderson (@parkerhendo))

Olmo Maldonado (@ibolmo) I'll go back through and add some tests for it.

[Parker Henderson (parkerhendo)]

Olmo Maldonado (@ibolmo) added some tests for the new paths.