Keep within_namespace as part of our internal api

[ericproulx]

Clarify within_namespace visibility; remove legacy namespace_start/namespace_end

Motivation

• Keep within_namespace as a private, internal helper for scoping settings safely.
• Remove references to legacy namespace_start/namespace_end to simplify internals and tests.

Changes

• within_namespace remains private in Grape::DSL::Settings and continues to be used internally by DSL methods (e.g., namespace, version, scope).
• Clean up and refactor related code/comments; adjust specs to exercise namespacing behavior without relying on legacy helpers.
• Minor updates in lib/grape/dsl/settings.rb and corresponding specs.

Impact

• No public API change; typical namespace usage is unaffected.
• If any external code was calling internal helpers directly, that usage is unsupported and may need to rely on public DSL (namespace, group, resources, etc.).

Tests

• Specs updated to reflect internal scoping behavior while keeping within_namespace private; removed legacy helper references.