feat(slimrpc): update all protocol binding implementations to align with spec

[Tehsmash]

Context

Closes part of #1786. Blocked by #1803.

Once the SLIMRPC custom protocol binding spec is updated to document the hierarchical config loading behaviour (#1803), all existing protocol binding implementations across languages must be brought into conformance.

Scope

Audit and update every SLIMRPC protocol binding implementation to ensure it:

• Implements the config file search order specified in feat(slimrpc-spec): document hierarchical config in custom protocol binding spec #1803
• Exposes the correct environment variable override names
• Handles missing config gracefully per the spec
• Passes any conformance tests defined as part of the spec update

Acceptance Criteria

• All language implementations (Python, Go, .NET, JS) audited against updated spec
• Any deviations corrected
• Conformance tests (if added to spec) pass for all implementations
• Changelog / migration notes written for any breaking changes