fix(loki sink): validate endpoint is an absolute URL

[xfocus3]

Summary

Closes #10980.

The loki sink's endpoint option is deserialized as a permissive UriSerde, which accepts incomplete URIs. A value with a missing or mistyped scheme (for example a bare host name) is accepted at configuration parse time, but then fails later — when the request URI is assembled via append_path — with an opaque error:

Configuration error. error=Sink "loki": invalid format

This gives the user no indication that the problem is the endpoint value.

This PR validates the endpoint when the sink is built (alongside the existing label validation) and returns a clear, actionable error instead.

Before

[sinks.loki]
type = "loki"
inputs = ["in"]
labels.host = "localhost"
endpoint = "something-with-a-typo-that-is-hard-to-notice"
encoding.codec = "logfmt"

Configuration error. error=Sink "loki": invalid format

After

Configuration error. error=Sink "loki": Invalid `endpoint`: ... is not an absolute URL. Expected a value with a scheme and host, such as "http://localhost:3100".

How did you test this PR?

• cargo check --lib --no-default-features --features sinks-loki — passes
• cargo test --lib --no-default-features --features sinks-loki loki::config — passes (added validates_endpoint unit test; existing valid_label_names still passes)
• cargo fmt --check — passes
• cargo clippy --lib --no-default-features --features sinks-loki -- -D warnings — passes

The fix matches the direction suggested by @jszwedko in the issue thread (checking for the needed URL parts at the configuration layer).

Change Type

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

Is this a breaking change?

• Yes
• No

A configuration with a scheme-less endpoint already failed to start (with an unclear error); it now fails with a clear one. Valid absolute-URL endpoints are unaffected.

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Closes: Configuration error. error=Sink "loki": invalid format #10980