Fix mod_h2 vs mod_http2 naming in Ubuntu HTTP/2 diagnostic + build script

docs/userguide/json-httpd-h2-userguide.md:
- Diagnostic script checked for mod_h2.so but Ubuntu 22.04+ ships
  mod_http2.so — caused false "compile from source" recommendation on
  systems that already have HTTP/2 available
- Fix Tests 2/3/recommendation to check both mod_http2.so and mod_h2.so
- a2enmod call updated: try 'http2' first (Ubuntu 22.04 name), fall back
  to 'h2' (older distros)
- IfModule guard changed from mod_h2.c to mod_http2.c in site config
- Verified platform updated: Ubuntu 22.04 LTS / Apache 2.4.52
- "When to use source compilation" note no longer cites Ubuntu 25.10

build_financial_service.sh:
- Add missing source files to gcc invocation:
  financial_benchmark_service_handler.c (D1 HTTP entry points)
  mcp_catalog_handler.c (D1 MCP catalog endpoint)
  axis2_json_secure_fault.c (D2 secure error responses)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>