fix: use posthog-java User-Agent for server-side runtime detection

[igormq]

Summary

• PostHog's runtime detection uses User-Agent patterns to determine if requests come from server-side or client-side SDKs
• Use posthog-java User-Agent for server-side runtime detection to enable proper library bucketing
• This allows users to start migrating to posthog-java
• Adds internal userAgent property to core PostHogConfig (@PostHogInternal) - not exposed to end users
• Adds getUserAgent() method that returns userAgent or falls back to sdkName/sdkVersion
• Server module sets userAgent to "posthog-java/{version}"

See: PostHog/posthog#45652
See: PostHog/posthog#45652 (comment)

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Split from original PR addressing contributor feedback to keep changes focused.

[marandaneto]

@PostHog/team-feature-flags relates to PostHog/posthog#45652 i think you'd know better if this is needed or not

[dustinbyrne]

Similar to what I said here, I think the flags service should include the posthog-server user agent for now. It's server-side, so we can roll that change out instantly.