Make URL and POST parameters immutable

[lovasoa]

Problem

URL parameters and form data could be overwritten by SET statements, making it impossible to access original request values after modification.

Solution

Request parameters (URL and POST) are now immutable. SET creates user-defined variables in a separate namespace that shadow request parameters during lookup.

Changes

Data Structure:

• RequestInfo now has three variable maps:
  • url_params (immutable, from URL query string)
  • post_variables (immutable, from form data)
  • set_variables (mutable, from SET statements) using RefCell for interior mutability

Variable Lookup:

• $var: checks set_variables → url_params (deprecation warning for post_variables)
• :var: checks set_variables → post_variables
• All SET statements write to set_variables only

API Simplification:

• stream_query_results_with_conn and related functions now take &RequestInfo instead of &mut RequestInfo

New Functionality:

• sqlpage.variables('set') returns only user-defined variables
• sqlpage.variables('get') returns only URL parameters (immutable)
• sqlpage.variables('post') returns only POST parameters (immutable)
• sqlpage.variables() merges all with SET taking precedence

Documentation:

• Updated extensions-to-sql.md with variable types and precedence rules
• Updated 20_variables_function.sql migration with new 'set' filter

Tests:

• Added it_works_immutable_variables.sql test
• All 151 tests passing (110 unit + 41 integration)

Breaking Changes

None. Existing code continues to work with identical behavior.