fix(ado): detect ADO sign-in HTML response in list and report auth error

[Copilot]

Summary

ado-aw list failed with Failed to parse definitions response as JSON ... the PAT is invalid or expired even when authenticating via the Azure CLI. ADO serves its AAD sign-in HTML page with a 203 Non-Authoritative Information status; because 203 is a 2xx, is_success() passed and the JSON parser then choked on HTML, producing a misleading, PAT-specific error.

Changes (src/ado/mod.rs):

• looks_like_ado_signin(status, content_type, body) — flags the sign-in response via 203 status, text/html content type, or an HTML body.
• ado_signin_error(auth) — auth-method-specific guidance: ****** CLI) advises az login + org/project access; PAT advises regenerating the token.
• list_definitions — checks the response before JSON parsing and returns the accurate auth error instead of a parse failure.
• Unit tests for 203, HTML content-type, HTML body, valid-JSON negative case, and per-auth messages.

Scope is limited to the reported list path; the helper is reusable for the other JSON-parsing call sites in src/ado/mod.rs as a follow-up.

Test plan

• cargo test --bin ado-aw ado:: (90 pass, including 5 new detection tests)
• cargo clippy --all-targets --all-features (clean)