fix: prevent memory leak in otel schema_version decode branch

[jbachorik]

What does this PR do?:
Guards the threadlocal.schema_version decode branch in otel_process_ctx_decode_payload against leaking the existing thread_ctx_config allocation. When data_out->thread_ctx_config is already non-NULL, the fix reuses the existing struct instead of allocating a new one and silently losing the old pointer. It also frees the old schema_version value before assigning a new one (handles duplicate keys in a single payload).

Motivation:
The schema_version branch at lines 808-812 always executed calloc + unconditional assign, meaning every re-decode (or any payload that contained both threadlocal.attribute_key_map and threadlocal.schema_version) would leak the previously allocated otel_thread_ctx_config_data struct. The attribute_key_map branch directly above it already used the correct null-guard pattern; this fix mirrors that pattern for schema_version.

Additional Notes:
The fix is symmetric with the existing attribute_key_map branch (lines ~790-800) which already checked !data_out->thread_ctx_config before allocating. No new allocation strategy is introduced — the same calloc + null-guard pattern is applied consistently.

How to test the change?:
Acceptance test scenarios (manual or via a C unit test harness):

• Decode a payload containing threadlocal.schema_version twice — second decode must not leak the first allocation
• Decode a payload with attribute_key_map first, then schema_version — thread_ctx_config must be allocated exactly once
• Decode a payload with schema_version first, then attribute_key_map — same single-allocation requirement
• Run under ASan/Valgrind with a payload that exercises both keys; expect zero leak reports

For Datadog employees:

• This PR doesn't touch any of that.
• JIRA: PROF-14142

[dd-octo-sts]

CI Test Results

Run: #24229769402 | Commit: e389034 | Duration: 26m 59s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

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Updated: 2026-04-10 06:59:16 UTC