feat: Add .NET Server AI SDK examples

[jsonbailey]

Summary

Stacked on #1. Adds the server-sdk-ai/ partition (LaunchDarkly.ServerSdk.Ai) following the EXAM layout.

Examples are sourced from dotnet-core PR #293 (AIC-2789) — each is a self-contained console app referencing the published NuGet packages with floating versions (8.* / 0.*), so no project-reference rewiring was needed.

Layout

server-sdk-ai/
├── README.md                         # index (Getting Started + Features)
├── frameworks/                       # provider integrations (was getting-started/ in #293)
│   ├── openai/chat-completions/
│   └── bedrock/converse/
└── features/                         # provider-agnostic SDK-surface examples
    ├── completion-config/
    ├── agent-config/
    └── judge-config/

Notes

• Per EXAM, provider integrations live under frameworks/ rather than getting-started/; the README still presents them under a Getting Started heading.
• Fixed the feature READMEs' cross-links (../../getting-started → ../../frameworks).
• Root README gains a .NET Server AI SDK row; CODEOWNERS gives team-ai-agents/team-ai-evals ownership of /server-sdk-ai/.
• Verified: completion-config and openai/chat-completions build clean against the floating versions.
• All files BOM-free.

Not included

• CI doesn't build/run the AI examples yet (they need provider keys to run). Follow-up if we want build-only coverage.

Jira: AIC-2801

[jsonbailey]

Per Ryan's feedback, switched from a generic build.yml to a per-slice workflow: server-sdk-ai.yml builds only the server-sdk-ai/ examples and is path-filtered to run only when that folder (or its workflow) changes. Supersedes the build.yml references in the description.