Fix flaky TestMuxHistoryOrdering test

[google-labs-jules]

Fixed a race condition in TestMuxHistoryOrdering where sending messages too quickly could cause the stream buffer to initialize incorrectly or timeout.

Changes:

• Split the message sending loop.
• Explicitly wait for the "Anchor" message (index 0) to be received before sending the rest.
• Retained the core test logic verifying that out-of-order messages (C, then B) are correctly reordered (B, then C).

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PR created automatically by Jules for task 4391622681074417945 started by @KCarretto

[google-labs-jules]

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2735    ±0	2735    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#69	✅	2735	2735	0	0	0	0	28.2s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2735	0	0	43.1s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch: random::string_impl::tests::test_string_uniform	3	39.4s	43.1s
eldritch: random::string_impl::tests::test_string_uniform	3	39.4s	43.1s
eldritch: random::string_impl::tests::test_string_uniform	3	39.4s	43.1s
eldritch-stdlib-tests: tests::test_all_tomes	3	5.5s	6.2s
eldritch-stdlib-tests: tests::test_all_tomes	3	5.5s	6.2s
eldritch-stdlib-tests: tests::test_all_tomes	3	5.5s	6.2s
eldritch: random::string_impl::tests::test_string_length	3	5.7s	6.0s
eldritch: random::string_impl::tests::test_string_length	3	5.7s	6.0s
eldritch: random::string_impl::tests::test_string_length	3	5.7s	6.0s
eldritch: sys::dll_reflect_impl::tests::test_dll_reflect_starlark	1	5.4s	5.4s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

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