fix(tracer): use atomic_store for writer->headers assignment

[realFlowControl]

Description

writer->headers is declared _Atomic but was being stored via a plain non-atomic assignment in dd_curl_set_headers, while dd_curl_reset_headers reads it with atomic_exchange. This replaces the plain write with atomic_store to match.

Note: the root cause of the crash in #3917 is not fully identified/fixed with this PR

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[datadog-official]

 

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 14 Pipeline jobs failed

DataDog/apm-reliability/dd-trace-php | test_extension_ci: [7.0]     

See error

2 failed tests: Enabling dynamic instrumentation and Installing a live debugger span decoration probe.

DataDog/apm-reliability/dd-trace-php | test_extension_ci: [7.2]     

See error

Assertion failed in zend_hash.c at line 712: Assertion `((ht)->gc.refcount == 1) || ((ht)->u.flags & (1<<6))' failed due to unexpected state of HashTable.

DataDog/apm-reliability/dd-trace-php | test_extension_ci: [7.3]     

See error

Assertion failed in zend_hash.c:903 during dynamic instrumentation. Error: `(zend_gc_refcount(&(ht)->gc) == 1) || ((ht)->u.flags & (1<<6))`

View all 14 failed jobs.

ℹ️ Info

No other issues found (see more)

🧪 All tests passed
❄️ No new flaky tests detected

🔄 Datadog auto-retried 3 jobs - 3 passed on retry

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 54.12% (-0.04%)

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[bwoebi]

I surely can approve this, but atomic_store only matters when it's sequenced relative to other operations.

This shouldn't fix anything at all.