Fix assign_swallows_filter_error, server error propagation and error_mode

assign_swallows_filter_error: add render_errors: true so strict_errors
is false and assign swallows the filter error as expected.

Server (remote-liquid.rb):
- Default error_mode to :strict (was defaulting to liquid-ruby's lax)
- Use strict_errors from render options for rethrow_errors (was always
  false, causing errors to be swallowed instead of raised)

With proper feature opt-outs (ruby_drops, ruby_types, drop_class_output,
lax_parsing, strict2_blank_body_errors, template_factory), JSON-RPC
adapter is down to 9 failures — all pre-existing:
- 4 filesystem error message format differences ('asset' vs 'template')
- 3 date filter formatting (raw Time#to_s instead of formatted)
- 2 resource limits (not enforced over JSON-RPC)

Ruby reference: 5010 passes, 0 failures, 274 skipped, level 1000/1000.
92 unit tests, 0 failures. All 5 verifiers pass.

Author: tobi

scripts/remote-liquid/remote-liquid.rb

@@ -304,7 +304,7 @@ def handle_compile(params)
       # Build parse options
       parse_options = {}
       parse_options[:line_numbers] = true if options["line_numbers"]
-      parse_options[:error_mode] = options["error_mode"]&.to_sym
+      parse_options[:error_mode] = (options["error_mode"] || "strict").to_sym
 
       # Parse template - capture parse errors in result, not as exception
       begin
@@ -346,11 +346,13 @@ def handle_render(params)
       registers = {}
       registers[:file_system] = @filesystems[template_id]
 
-      # Build context - never rethrow errors, we capture them
+      # Build context — rethrow errors when strict_errors is true
+      # so the adapter can match them against expected error patterns
+      strict_errors = options["strict_errors"] || false
       context = Liquid::Context.build(
         static_environments: unwrapped_env,
         registers: Liquid::Registers.new(registers),
-        rethrow_errors: false
+        rethrow_errors: strict_errors
       )
 
       # Handle time freezing

specs/liquid_ruby/filter_argument_semantics.yml

@@ -57,6 +57,7 @@ specs:
 - name: assign_swallows_filter_error
   template: 'before{% assign v = "x" | truncate: nil %}after[{{ v }}]'
   expected: beforeafter[]
+  render_errors: true
   complexity: 500
   hint: |
     A filter that raises during the expression of an {% assign %} does not