feat(api): allow signup without creating a session (#2586)

[YadneshTeli]

What kind of change does this PR introduce?

Feature.

Resolves #2586 (and relates to supabase/supabase-flutter#1063).

What is the current behavior?

Today the POST /signup endpoint creates and returns a session whenever the user is auto-confirmed (or when confirmation is disabled project-wide). A session is only omitted when email/phone confirmation is required.

Client applications reacting to AuthChangeEvent.SIGNED_IN or needing to perform post-signup preparatory work before establishing an active session had no opt-in mechanism to sign up without automatically generating session tokens.

What is the new behavior?

• Adds an optional create_session boolean property (defaults to true) to POST /signup request parameters.
• When create_session: false is supplied, the user is created and auto-confirmed (if autoconfirm is enabled), but no session/refresh tokens are generated and no login metering is recorded. The endpoint returns the models.User object directly with 200 OK.
• Retains 100% backwards compatibility: when create_session is omitted or set to true, the behavior is completely unchanged.
• Adds comprehensive unit and integration tests in internal/api/signup_test.go.
• Updates openapi.yaml schema and response documentation.
• Improves build compatibility across platforms for socket reuse and signal handling tests.

Additional context

Verified against live PostgreSQL and test suites (go test -v ./internal/api -run TestSignup).