fix(api-client): Relax 'context' field schema in BaseEventSchema

[sentry]

The BaseEventSchema in packages/mcp-core/src/api-client/schema.ts defined the context field (the legacy 'extra' field) as z.record(z.string(), z.unknown()).optional().

This schema was too strict, as Sentry's API can return non-object shapes (such as arrays or null) for this field on certain events. This led to Zod validation failures and Event failed schema validation errors being logged.

This change relaxes the schema for the context field to z.unknown().optional() to correctly handle the arbitrary nature of this legacy field, preventing validation errors when fetching events from the Sentry API.

Fixes MCP-SERVER-G93

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[sentry-warden]

Relaxing context to unknown crashes formatting and fails typecheck

Widening context to z.unknown() lets non-object values through validation, but formatContext() still calls Object.keys/Object.entries on it, so arrays/primitives throw at runtime and the strict tsc job rejects unknown there.

Evidence

• Hunk changes BaseEventSchema.context from z.record(z.string(), z.unknown()).optional() to z.unknown().optional(), so inferred Event["context"] is now unknown | undefined.
• formatEventOutput() always calls formatContext(eventToRender.context) (formatting.ts).
• formatContext() does Object.keys(context) and Object.entries(context) with no plain-object guard; non-null non-objects (arrays, numbers, strings, true) throw, and TypeScript under strict: true rejects those calls on unknown.
• PR motivation is that Sentry can return non-object context shapes; CI runs pnpm -w run tsc in .github/workflows/test.yml.

_{Identified by Warden · code-review · EKN-GGF}