Fix stack trace frame names to show enclosing function instead of callee

[stephenamar-db]

Summary

• Rework Error.formatError to label each stack frame with the enclosing function (the function whose body contains that position) instead of the callee (the function being called). This matches the convention used by Java/Python stack traces.
• The fix scans frames outermost-to-innermost to compute the enclosing scope for each position. Builtin calls (std.*), comprehensions, and default parameter evaluations are transparent and don't create new scopes. Consecutive same-scope frames are deduplicated.
• Add regression test for cross-file max-stack-frames error (error.max_stack_cross_file).

Before

sjsonnet.Error: [search] Max stack frames exceeded.
    at [search].(b.jsonnet:5:37)
    at [std.objectHas].(b.jsonnet:5:25)
    at [method].(a.jsonnet:5:23)
    at [m].(a.jsonnet:9:2)
    at [<root>].(a.jsonnet:1:7)

After

sjsonnet.Error: [search] Max stack frames exceeded.
    at [method].(b.jsonnet:5:37)
    at [m].(a.jsonnet:5:23)
    at [<root>].(a.jsonnet:9:2)

Each [name] now reflects the function you are in, not the function you called.

Test plan

• All existing JVM tests pass (140/140)
• Updated 6 golden files to match corrected trace format
• Added new regression test error.max_stack_cross_file covering cross-file recursive max-stack error