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
• 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

Aligns the Ruby AI SDK's usage tracking with the updated spec from sdk-specs PR #132. Three categories of changes:

1. Renamed usage tracking event key from $ld:ai:config:function:single to $ld:ai:usage:completion-config, matching the new unified $ld:ai:usage:* namespace. (This SDK only has the single completion config method — no judge, agent, or createChat methods — so only one key needed updating.)

2. Added SDK info tracking on initialization ($ld:ai:sdk-info). When Client is constructed, it fires a single tracking event with SDK name, version, and language using an anonymous internal context. This satisfies spec requirement 1.2.2.1 and ensures data is captured even if the config tracker is never used. SDK identity constants are kept in a dedicated sdk_info.rb module (similar to sdkInfo.ts in JS and sdk_info.py in Python).

3. Renamed config to completion_config to align with the updated spec method naming. The old config method is preserved as a deprecated proxy that emits a warning and delegates to completion_config.

Human review checklist

• Verify event key string $ld:ai:usage:completion-config exactly matches the spec — a typo here would silently break reporting
• Verify the anonymous context (LDContext.create(key: 'ld-internal-tracking', anonymous: true)) matches spec requirement 1.2.2.1
• Confirm that INIT_TRACK_CONTEXT being created at module load time (outside class Client) is acceptable — it calls LDContext.create when the module is first loaded
• Confirm the deprecation approach for config (emitting warn on every call) is acceptable for existing consumers — this could be noisy if callers haven't migrated yet

Additional context

• Link to Devin run: https://app.devin.ai/sessions/9c25c81dfcc347a08bdd5f168ae59725
• Requested by: @jsonbailey

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[jsonbailey]

There should be a break between methods here.

[devin-ai-integration]

Added a blank line between the methods.

[jsonbailey]

In other sdk's we kept this information in an sdk info / meta type file so it could be used in other places if needed.

[devin-ai-integration]

Extracted SDK info into a dedicated lib/server/ai/sdk_info.rb module with SDK_NAME and SDK_LANGUAGE constants (similar to sdkInfo.ts in JS and sdk_info.py in Python). The client now references those constants instead of hardcoded strings.