Expose secrets from executor to function context

[jmarbutt]

Problem

Secrets sent by the Inngest executor in CallRequestPayload.Secrets are parsed but never exposed to functions. The SDK has a _secrets dictionary that's only populated via RegisterSecret(), but secrets from the executor are ignored.

Location: Inngest/InngestClient.cs:1110-1201 (HandleCallRequest)

The payload.Secrets field is never read or passed to InngestContext.

Expected Behavior

Functions should be able to access secrets via context.GetSecret("key") or similar API, where secrets are sourced from:

1. The executor's CallRequestPayload.Secrets
2. Locally registered secrets via RegisterSecret()

SDK Spec Reference

This blocks the SDK_SPEC "secrets" capability.

Suggested Implementation

1. Add a Secrets property to InngestContext
2. Merge payload.Secrets with locally registered secrets when building context
3. Provide a GetSecret(string key) method

Priority

Medium - required for full SDK spec compliance.