Allow disabling context-sensitivity for specific functions

[Copilot]

Functions called with many different contexts can cause analysis blowup. This adds a way to disable context-sensitivity for specific functions at the MCP level, returning an empty context list [] for all call sites of the annotated function.

Three mechanisms supported

• C attribute (no source modification to definition needed):

  int f(int x) __attribute__((goblint_context("no-context")));
  int f(int x) { ... }

• Config option (no source changes required):

  --set ana.context.no_fun[+] f
  

• Per-function annotation config:

  --set annotation.goblint_context.f[+] no-context
  

Changes

• src/analyses/mCP.ml: context function now short-circuits to [] when the function has the no-context attribute or appears in ana.context.no_fun, making it context-insensitive across all activated analyses
• src/config/options.schema.json: Added "no-context" to goblint_context attribute enum; added ana.context.no_fun string-list option
• docs/user-guide/annotating.md: Documented new no-context attribute value
• tests/regression/90-context/: Regression tests for all three mechanisms