fix(sdk): add bounded timeout to validateHealthyPlatformConnection

[marythought]

Context

validateHealthyPlatformConnection (the gate run by WithConnectionValidation) calls checkPlatformHealth(context.Background(), …) with no deadline. If a custom http.Client has no timeout configured, sdk.New() can block indefinitely on a network stall.

Surfaced in #3412 (CodeRabbit major-priority review).

Proposal

Wrap the call in a bounded context. Open question: hard-coded 5s, configurable via a new WithConnectionValidationTimeout(d) option, or derive from an existing config?

Acceptance

• sdk.New() cannot block longer than the configured timeout when validation is enabled
• Test: covers the timeout path against a hanging server (httptest.Server with a slow handler)
• Documented in WithConnectionValidation doc comment