fix: update stale format_c golden files after PR #1015 changed code behavior

[He-Pin]

Summary

PR #1022 added format_c error test files intended as part of PR #1015 %c validation coverage. However, PR #1022 CI ran at 08:07 UTC, before PR #1015 was merged at 16:51 UTC. The golden files were generated from code without %c rejection, so they contained success output instead of error messages.

Two minutes after #1015 merged, #1022 was squash-merged. The stale golden files entered master without a CI re-run on the combined state.

Timeline

08:07  PR #1022 CI runs, master has NO %c rejection code
       "%c" % ""   outputs ""     golden correct at this time
       "%c" % "AB" outputs "AB"   golden correct at this time
       CI all green

16:51  PR #1015 merged, %c rejection code enters master
16:53  PR #1022 merged, stale golden files enter master with no CI re-run

After  All PRs branch from master with both %c rejection and stale goldens
       "%c" % ""   errors, golden "" is now wrong
       "%c" % "AB" errors, golden "AB" is now wrong

Fix

Update both golden files to contain the actual sjsonnet.Error output:

File	Before stale	After correct
error.format_c_multichar.jsonnet.golden	"AB"	sjsonnet.Error: [std.format] %c expected 1-sized string got: 2
error.format_c_empty_string.jsonnet.golden	""	sjsonnet.Error: [std.format] %c expected 1-sized string got: 0

Implementation behavior comparison

Behavior verified with go-jsonnet and jrsonnet latest local binary, jrsonnet 0.5.0-pre99. Both implementations reject empty and multi-character strings for %c, and accept single Unicode code points.

Expression	go-jsonnet 0.21.0	jrsonnet 0.5.0-pre99	Expected sjsonnet behavior
std.format("%c", "")	exit 1, %c expected 1-sized string got: 0	exit 1, %c expected 1 char string, got 0	exit 1, %c expected 1-sized string got: 0
std.format("%c", "AB")	exit 1, %c expected 1-sized string got: 2	exit 1, %c expected 1 char string, got 2	exit 1, %c expected 1-sized string got: 2
std.format("%c", "A")	exit 0, "A"	exit 0, "A"	exit 0, "A"
std.format("%c", "世")	exit 0, "世"	exit 0, "世"	exit 0, "世"
std.format("%c", 127757)	exit 0, "🌍"	exit 0, "🌍"	exit 0, "🌍"

Verification

./mill "sjsonnet.jvm.2_13_18.test" -- "*FileTests*"
Tests: 102, Passed: 102, Failed: 0

rtk ./mill "sjsonnet.jvm[3.3.7]".test.testOnly sjsonnet.FileTests
Tests: 3, Passed: 3, Failed: 0

rtk ./mill "sjsonnet.jvm[3.3.7]".test
SUCCESS

Test plan

• FileTests.new_test_suite passes with corrected golden files
• Full JVM Scala 3.3.7 test suite passes locally
• go-jsonnet and jrsonnet behavior compared for %c empty, multi-character, single ASCII, single Unicode, and numeric codepoint cases
• CI green on all 4 build targets: js, jvm, native, wasm

[He-Pin]

@stephenamar-db This one need to be merged first.