Backport #24553 to 7.82.x: return freed memory to the OS with malloc_trim after each run

[piochelepiotr]

What does this PR do?

Backports #24553 to the 7.82.x branch. Calls glibc malloc_trim(0) after every kafka_consumer check run (in a finally), so per-arena free memory is returned to the OS each cycle. It is resolved once at import via ctypes and is a no-op on platforms without it (macOS, musl).

Motivation

librdkafka runs roughly one thread per broker, and on high-core hosts glibc spreads allocations across many per-thread arenas (default cap 8 x ncpu). glibc only ever returns memory from the top of an arena's heap and never coalesces free chunks across arenas, so the transient allocations each run leaves behind accumulate as retained-but-unused memory and agent RSS ratchets upward. malloc_trim(0) walks the arenas and hands their free memory back to the OS, keeping RSS bounded without requiring a MALLOC_ARENA_MAX environment variable. Overhead is negligible for this check (runs are network-bound at several seconds).

The automated backport to 7.82.x failed on a cherry-pick conflict (7.82.x lacks unrelated base64/marshal imports and constants added to master by other commits). Resolved manually, keeping only this PR's changes.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[temporal-github-worker-1]

Backport evaluation: View Slack thread

[dd-octo-sts]

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• Overall Coverage: 91.69%

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