fix(dnstap source): don't delete the socket file during config validation

[xfocus3]

Summary

Closes #25513.

vector validate -d (documented as a dry-run) deleted the Unix socket file of any running dnstap source in mode: unix, killing the source's accept loop.

Root cause

build_framestream_unix_source() (src/sources/util/framestream.rs) removed any existing file at the socket path and bound the listener eagerly, before constructing the source future:

match fs::metadata(&path) { Ok(_) => { fs::remove_file(&path)?; } ... };
let listener = UnixListener::bind(&path)?;

This is fine for a real startup, but vector validate instantiates all components (via TopologyPiecesBuilder::build()) to validate them — which runs this destructive code against the live socket of a separate, already-running Vector instance.

Fix

Move the socket setup (stale-file removal, bind, buffer-size sockopts, permission changes) into the returned source future, so it only executes when the source actually starts — not when it's built/validated. This matches how the other unix sources already work (util/unix_stream.rs, util/unix_datagram.rs bind inside Box::pin(async move { ... })).

Build/validation errors are now logged and surfaced as a source failure (Err(())) inside the future, consistent with the other unix sources.

How did you test this PR?

• Added a regression test (build_unix_source_does_not_remove_existing_socket) that creates a file at the socket path, builds the source, and asserts the file still exists after build (it is only managed when the future runs).
• All existing framestream tests pass: cargo test --lib --no-default-features --features sources-dnstap sources::util::framestream::test → 12 passed.
• cargo check, cargo clippy -- -D warnings, and cargo fmt --check pass for the sources-dnstap feature.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve validation of invalid socket_file_mode

With this check now inside the returned source future, build_framestream_unix_source returns Ok during component construction for configs such as a dnstap unix source with socket_file_mode = 0o600; vector validate will therefore pass a config that immediately fails when the source starts. The destructive socket operations need to be deferred, but this non-destructive mode validation should still happen during build/config validation or in a dedicated config validator.

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