fix(fluent-bit): align app.kubernetes.io/version label with image tag

[bhataprameya]

What this does

The app.kubernetes.io/version label is hardcoded to .Chart.AppVersion in
fluent-bit.labels. When a user overrides image.tag, the deployed image and
the version label diverge — the label keeps reporting the chart's appVersion
while the pod runs the overridden tag.

This change resolves the label to the image actually deployed via a new
fluent-bit.version helper that returns image.tag when set, and falls back to
.Chart.AppVersion when the tag is unset or the - sentinel is used (mirroring
the existing fluent-bit.image tag handling). The helper is also used by the
test-connection pod for consistency.

Behaviour

image.tag	app.kubernetes.io/version
unset (default)	.Chart.AppVersion (unchanged)
"-" (digest-only)	.Chart.AppVersion
e.g. 4.0.8	4.0.8

Default behaviour is unchanged; the label only changes when image.tag is
overridden.

Checklist

• CHANGELOG.md entry added under [UNRELEASED]
• Chart version bumped (0.57.6 → 0.57.7)
• README regenerated (version bump only; no values changes)
• helm lint passes
• Commits signed off (DCO)

Closes #

[stevehipwell]

@bhataprameya you shouldn't be changing the chart tag to change the Fluent Bit version, you should be updating the chart. A Helm chart should be treated as a complete package and just because you can modify the tag to use a different version doesn't mean that it's supported or that you should do it.