VPAAMP-558: restore per-host curl share locks; fix non-store USERDATA=NULL

[pstroffolino]

Issue 1 (medium risk -- per-host DNS/SSL lock granularity):

Embed CurlDataShareLock mShareLock directly in curlstorestruct as a
value member. CURLSHOPT_USERDATA is now set to &CurlSock->mShareLock
so each CDN hostname gets its own three-way mutex (DNS / SSL / general),
restoring the parallelism lost by the VPAAMP-139 single-static-lock fix.

Safety: every cleanup path (RemoveCurlSock, ~CurlStore,
FlushCurlSockForHost) follows the order:
1. curl_easy_cleanup all queued handles (no further easy-hdl callbacks)
2. curl_share_cleanup(mCurlShared) (share teardown; lock may fire)
3. SAFE_DELETE(CurlSock) (destroys embedded mShareLock)
The embedded lock is therefore always alive when curl_share_cleanup
calls it, eliminating the original use-after-free without leaking memory.

Removes: CurlStore::mSharedCurlLock (static class member, no longer
needed) and the old large IMPORTANT comment block.

Issue 2 (low risk -- non-store path USERDATA=NULL inconsistency):

Replace static std::mutex gCurlShMutex with static CurlDataShareLock
gCurlShLock. CurlInit (non-store path) now passes &gCurlShLock as
CURLSHOPT_USERDATA instead of NULL, giving DNS/SSL/general lock
granularity parity with the store path. The defensive else branches
in curl_lock/unlock_callback are updated accordingly.

Test updates (CurlStoreTests.cpp):

T1 CreateCurlStore_UserDataDiffersAcrossHosts
Flipped from SAME to DIFFERENT: two hostname entries must now
receive distinct CURLSHOPT_USERDATA pointers (per-host lock).

T2 LockCallback_WorksWhileEntryIsAlive
Replaced: captures callbacks while entry is alive (count > 0) and
invokes them; verifies the embedded lock is functional.

T3 ShareCleanup_CalledOnEviction
Replaced: verifies that curl_share_cleanup is called with the
correct CURLSH handle when RemoveCurlSock evicts an entry, testing
the cleanup-order invariant that guarantees lock safety.

T4 LockCallback_HandlesAllLockDataTypes
Unchanged (still exercises DNS/SSL/generic branches).

[Copilot]

Pull request overview

Restores per-host libcurl share locking in CurlStore to regain DNS/SSL cache parallelism (lost in the prior single-static-lock fix), and fixes the non-store path to pass a valid CURLSHOPT_USERDATA pointer instead of NULL.

Changes:

• Embed CurlDataShareLock as a value member in curlstorestruct and pass &CurlSock->mShareLock as CURLSHOPT_USERDATA for per-host lock granularity.
• Replace the non-store global std::mutex fallback with a process-lifetime CurlDataShareLock and pass it via CURLSHOPT_USERDATA in CurlInit.
• Update L1 tests to validate per-host distinct USERDATA pointers and the expected curl_share_cleanup eviction behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
downloader/AampCurlStore.h	Adds per-host embedded share lock to the curl store entry struct; removes the static shared lock.
downloader/AampCurlStore.cpp	Switches share USERDATA to per-entry lock; introduces process-lifetime lock for non-store path; adjusts callback fallback locking.
test/utests/tests/AampCurlStore/CurlStoreTests.cpp	Updates regression tests to assert distinct per-host USERDATA and validate eviction cleanup ordering.