caddyhttp: return 413 for oversized body placeholders

[cyphercodes]

Fixes #7691

Summary:

• read request body placeholders with error handling instead of ignoring read errors
• convert http.MaxBytesError from request_body max_size into an HTTP 413 handler error
• propagate placeholder errors through templates and vars/vars_regexp matchers
• add regression coverage for oversized {http.request.body} usage

Tests:

• go test ./modules/caddyhttp ./modules/caddyhttp/templates -count=1
• go test ./caddytest/integration -run 'TestRespondWithJSON|TestRequestBodyPlaceholderRespectsMaxSize' -count=1
• git diff --check

Assistance Disclosure

This PR was implemented with assistance from Hermes Agent (OpenAI Codex/GPT-5.5). The changes were reviewed and verified with the targeted tests listed above.

[francislavoie]

I agree with the concept, but the implementation might have too many side effects. Changing how errors for all placeholders are handled has far-reaching effects. Needs deeper analysis.