fix: Improve usage reporting

[jsonbailey]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

• Implements AIC-1636
• Spec: https://github.com/launchdarkly/sdk-specs/pull/132 and https://github.com/launchdarkly/sdk-specs/pull/135
• JS reference implementation: fix: Improve usage reporting js-core#1108
• Python reference implementation: fix: Improve usage reporting python-server-sdk-ai#92

Describe the solution you've provided

Updates the .NET AI SDK's usage tracking to align with the updated spec. Key changes:

1. Renamed usage tracking event key from $ld:ai:config:function:single to $ld:ai:usage:completion-config, following the new $ld:ai:usage:* naming convention from the spec.

2. Added SDK info tracking on initialization ($ld:ai:sdk:info). When LdAiClient is constructed, it fires a tracking event with aiSdkName, aiSdkVersion, and aiSdkLanguage using an anonymous context (kind ld_ai, key ld-internal-tracking). SDK metadata lives in a new SdkInfo static class (SdkInfo.cs) for reuse across the package.

3. Renamed Config method to CompletionConfig on both ILdAiClient and LdAiClient. The old Config method is preserved but marked [Obsolete] and proxies to CompletionConfig.

Note: The .NET SDK does not currently have agent, judge, createChat, or createJudge methods, so only the single completion-config usage event key needed updating.

Human review checklist

• Verify event key strings ($ld:ai:sdk:info, $ld:ai:usage:completion-config) exactly match the spec
• Verify the anonymous context (kind ld_ai, key ld-internal-tracking) matches spec requirement 1.2.2.1
• Confirm SdkInfo.Version = "0.9.1" with // x-release-please-version comment is compatible with this repo's release-please configuration
• Confirm the breaking ILdAiClient interface change (adding CompletionConfig) is acceptable for a pre-release SDK
• Verify that SDK info tracking firing per LdAiClient instantiation (rather than once globally) aligns with spec intent

Additional context

• Link to Devin run: https://app.devin.ai/sessions/6d9d054b9f0049b2987d755f9b7386bb
• Requested by: @jsonbailey
• Local build/test verification was not possible due to missing dotnet SDK on dev machine; relying on CI for verification

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