fix: increase batch payload size limit from 32KB to 5MB

[cat-ph]

Summary

• The PHP SDK enforces a 6-year-old 32KB batch payload size limit across all three consumers (ForkCurl, LibCurl, Socket), silently dropping batches that exceed it
• Any non-trivial batch with feature flags or rich properties can hit this easily
• The PostHog /batch/ endpoint accepts up to 20MB
• The Python SDK uses 5MB, Ruby uses 500KB, JS has no client-side limit
• This bumps the PHP limit to 5MB to match Python

Changes

• lib/Consumer/ForkCurl.php - 32 * 1024 -> 5 * 1024 * 1024
• lib/Consumer/LibCurl.php - same
• lib/Consumer/Socket.php - same

Test plan

• Existing tests pass (no tests assert the 32KB limit on master)
• Verified >32KB batch (~55KB) sends successfully against local PostHog instance

[cat-ph]

I think this is probably a simpler solution for #31 than #82 and #83 since 5mb gives quite a bit more leeway than 5kb, I tested this against localhost with >32kb and it seems to go through just fine 🤔

[marandaneto]

2 things to consider

• move 5 * 1024 * 1024 to a const so its computed at compile time instead of runtime for every call
• maybe the payload is small because PHP is single threaded and will block the request until its finished (is that the case?), and 5MB takes much longer to upload than 32KB, we might consider something smaller?