feat: variable discovery and validation improvements

[httpdss]

Closes #94

• Defaults from environment variables via env or default_from_env in variable schema.
• Type coercion: boolean, integer, float/number; string by default.
• Validation: enum, regex/pattern, min/max numeric bounds.
• Required handling: in non-interactive mode, missing required variables raise a clear error.
• Tests added for env defaults and coercion/validation.

All tests pass (95).

[codecov]

Codecov Report

❌ Patch coverage is 61.11111% with 28 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@4796e57). Learn more about missing BASE report.
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
struct_module/template_renderer.py	61.11%	18 Missing and 10 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #104   +/-   ##
=======================================
  Coverage        ?   65.06%           
=======================================
  Files           ?       18           
  Lines           ?     1251           
  Branches        ?      223           
=======================================
  Hits            ?      814           
  Misses          ?      372           
  Partials        ?       65           

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