fix(registry): handle Poll::Pending from sparse registry source queries

[rconnol]

Issue

Title: cargo outdated panics with sparse private registries: "Source should be ready"

Summary

cargo outdated panics (v0.17.0) or bails (v0.18.0) when used with a project that depends on a sparse private registry.

Details

When querying a sparse registry source, source.query_vec() returns Poll::Pending on the first call because sparse registries (HttpRegistry) fetch package metadata on-demand via HTTP. The first call initiates the async request and returns Pending — the data isn't available until block_until_ready() is called to drive the HTTP response to completion.

The current code in find_update() calls block_until_ready() once before query_vec(), which is a no-op for sparse registries since no requests are in flight yet. When query_vec() then returns Poll::Pending, it is treated as a fatal error.

Cargo itself handles this with a poll loop:
https://github.com/rust-lang/cargo/blob/60400187f/src/cargo/core/resolver/errors.rs#L479-L488

Steps to Reproduce

1. Configure a sparse private registry in ~/.cargo/config.toml:

   [registries.my-registry]
   index = "sparse+https://my-registry.example.com/crates/cargo/"

2. Have a project with a dependency from that registry
3. Run cargo outdated

v0.17.0

thread 'main' panicked at src/cargo_ops/temp_project.rs:404:18:
Source should be ready

v0.18.0

Error: Querying source was unexpectedly pending

Expected Behavior

cargo outdated should query the sparse registry successfully and report outdated dependencies.

---

PR

Title: fix: handle Poll::Pending from sparse registry source queries

Summary

• Use cargo's poll loop pattern to correctly handle Poll::Pending from sparse registry sources
• Remove redundant block_until_ready() call that was a no-op for sparse registries

Details

Sparse registries (sparse+https://...) fetch package metadata on-demand via HTTP. The first
query_vec() call initiates the request and returns Poll::Pending. The fix replaces the
single query + bail with the same poll loop cargo itself uses:

https://github.com/rust-lang/cargo/blob/60400187f/src/cargo/core/resolver/errors.rs#L479-L488

Test plan

• Tested locally against a project with sparse private registry dependencies
• just ci passes