fix(env): use mpsc for autodiscovery events

[omribz156]

Summary

Changes autodiscovery event delivery from a bounded broadcast channel to a bounded MPSC channel. Sends are now awaited, so a burst of autodiscovery events applies backpressure instead of letting a lagging broadcast receiver miss events.

This also updates the provider receiver type, hands out the single receiver through the existing subscribe() API, and adds a local-provider regression test that uses a capacity-1 channel with two config events.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

How did you test this PR?

• cargo fmt --check
• git diff --check
• cargo metadata --no-deps --format-version 1

I attempted cargo test -p saluki-env autodiscovery::providers::local --lib, but this Windows workspace is missing the MSVC linker / Windows SDK libraries (link.exe, kernel32.lib, etc.), so the command fails while compiling dependency build scripts before it reaches Saluki code.

This was implemented with Codex assistance, with the final diff reviewed against the issue scope and the existing autodiscovery provider tests.

References

• Closes: Change autodiscovery channel to MPSC to avoid loss. #1605

[tobz]

While this PR is superficially correct, the implementation misses an important aspect: only one subscriber can exist now, and only one ever, not even just "at most one."

What we need to do instead is actually keep track of the separate subscriptions, creating an MPSC channel for every subscription and holding on to the senders. Every time we go to send, we'd use the result of the send -- did it succeed or is the receiver gone? -- and use that to influence whether or not we remove the attached sender from the list before forwarding the next event.

[omribz156]

Thanks, fixed in the latest push.

Changed both autodiscovery providers to keep a list of per-subscriber mpsc::Senders. Each subscribe() now creates its own bounded channel, and event delivery fans out to all current subscribers while pruning senders whose receivers have gone away. I also added local-provider coverage for fan-out plus closed-receiver pruning.

Verified with:

• cargo fmt --check
• cargo metadata --no-deps --format-version 1
• git diff --check

I also tried cargo test -p saluki-env autodiscovery::providers::local --lib, but this Windows environment fails earlier in process-memory with cannot find type StatSource in this scope, before the focused autodiscovery tests can run.

This follow-up was Codex-assisted, with the final diff reviewed before pushing.

[datadog-datadog-prod-us1-2]

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 1 Pipeline job failed

DataDog/saluki | check-fmt     

🔧 Fix in code (Fix with Cursor).

Formatting issues in autodiscovery.rs: unexpected use of 'use std::sync::Arc' which is already imported.

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[tobz]

Looks like all that's left is fixing the formatting lint and then this should be good to go.