Add stdlib tests for Exception module

[Copilot]

lib/Exception.fram had no accompanying stdlib tests. Adds test/stdlib/Exception.fram covering the core behavioral properties of hException.

Changes

• test/stdlib/Exception.fram — six test cases:

  • normal return passes through — handler not invoked when no raise occurs
  • raise triggers handler — handler receives the raised value and its result is returned
  • raise does not resume — continuation after raise is abandoned
  • inner exception caught by inner handler — capability identity determines which handler fires
  • outer exception skips inner handler — raise on an outer capability passes through inner handler
  • exception inside state handler — nesting with hState: exception result replaces the state computation's return value

• test/stdlib/TestAll.fram — adds import Exception to include the new tests in the suite

let _ = testCase "raise does not resume" (fn _ =>
  let executed = ~mut.ref False in
  let _ = handle exc with hException (fn _ => ()) in
    exc.raise ();
    executed := True
  in
  assertFalse executed.get)

let _ = testCase "outer exception skips inner handler" (fn _ =>
  let result =
    handle exc with hException (fn _ => "outer") in
    handle _ with hException (fn _ => "inner") in
    exc.raise "error"
  in
  assertEqS result "outer")

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[ppolesiuk]

The tests are already included in the target branch, so this PR can be closed now.