[analysis_server] Infer nullable type in 'Add type annotation' when assignment isn't unconditional

[hkarmoush]

Summary

The Add type annotation fix/assist (triggered e.g. by prefer_typing_uninitialized_variables) infers an uninitialized variable's type from assignments found anywhere in the rest of the enclosing block, without regard to whether those assignments are guaranteed to run. For a variable only assigned inside a loop body, a single-branch if, or a try block, that code might not execute, so the variable could still hold its implicit initial value, null, at the point of use — but the fix inferred a non-nullable type anyway.

int? f() {
  var result;
  for (var i in [1, 2, 3]) {
    result = i;
  }
  return result; // fix incorrectly inferred `int`, should be `int?`
}

This adds a conservative check for whether at least one of the remaining statements is guaranteed to assign the variable — accounting for if/else (both branches), try/finally, do-while (body always runs once), labeled statements, and immediately-invoked closures — and makes the inferred type nullable when it isn't.

Fixes #64015

Test plan

• Added regression tests for the issue's for-each loop, single-branch if, and try/catch cases, plus counterexamples (if/else, do-while, try/finally) that must remain non-nullable
• pkg/analysis_server/test/src/services/correction/fix/add_type_annotation_test.dart — 43/43 pass
• pkg/analysis_server/test/src/services/correction/assist/add_type_annotation_test.dart — 75/75 pass
• pkg/analysis_server/test/src/services/correction/test_all.dart (full correction suite) — 6385/6385 pass, no regressions
• dart format / dart analyze clean on changed files

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