feat(client): opt-in connection check on Qdrant::build()

[ameyypawar]

Fixes #258.

Problem

gRPC connections are established lazily, so Qdrant::build() returns Ok even when the server is unreachable — a connection failure only surfaces on the first API call. build() already runs a health check for the default compatibility check, but a connection error there is silently discarded (.ok()), so there is no way to detect an unreachable server up front.

Change

Adds an opt-in check_connection flag on QdrantConfig:

let client = Qdrant::from_url("http://localhost:6334")
    .check_connection()
    .build()?; // returns Err if the server can't be reached

• New check_connection: bool field (default false), a check_connection() builder method, and a set_check_connection() setter.
• Qdrant::new reuses the existing health-check path; when check_connection is set, the health-check error is returned instead of swallowed.
• Default behavior is unchanged — with the flag unset, build() stays lazy exactly as before, so this is non-breaking.

Tests

Two server-free unit tests (connection refused on a closed local port):

• default build succeeds when the server is down (the lazy path), and
• check_connection() build fails eagerly when the server is down.

These run under the existing test.yml (cargo test --all); no new workflow is added.

Verification

• cargo fmt --all -- --check — clean
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean
• cargo test --lib --all-features — passes (the 3 failing tests require a running Qdrant instance on :6334 and are unrelated to this change)

Note on #259

#259 also targets #258, via a separate connect() method. This PR instead makes the check opt-in on build(), reusing the existing health-check machinery without adding a new public method beyond the config toggle. Happy to align with whichever direction you prefer.

---

All Submissions

• Targets the dev branch.
• Followed the guidelines in the Contributing document.
• Checked for other open PRs for the same change (see the note on feat: add connect method to verify gRPC connectivity #259 above).

New Feature Submissions

1. Does your submission pass tests?
2. Have you formatted your code with rustfmt?
3. Have you checked your code with cargo clippy --all --all-features?

Changes to Core Features

• Added an explanation of what the change does and why.
• Added new tests for the change.
• Ran tests locally.

[timvisee]

Thanks! I like the way this is implemented.