fix(logging): harden sandbox log delivery under load

[krishicks]

Summary

Compress sandbox log pushes and split batches below the gateway decode limit so an oversized request cannot wedge the reconnect loop. Drop only individually undeliverable lines and report those losses to the gateway.

Track channel, reconnect-buffer, and oversized-line losses cumulatively. Preserve gateway accounting across RPC reconnects, reset it with sandbox teardown, and meter both sandbox-side and ingest-side drops.

Related Issue

Refs #1055

Changes

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[johntmyers]

Review findings

• [P1] Do not require gzip before every gateway can accept it. The supervisor unconditionally enables request compression (log_push.rs:307-315), while gateway support is introduced only in this PR (multiplex.rs:273-275). A newer supervisor connecting to an older gateway or replica gets an unsupported-compression error, then enters the reconnect/backoff path and eventually drops logs. Please either land server-side acceptance before enabling client compression, negotiate the capability, or retry uncompressed when compression is rejected.

• [P1] Requeue or account for batches rejected by the stream channel. send_logs returns false as soon as push_tx.send(...) fails, discarding the failed request and any remaining split groups (log_push.rs:233-257). Its callers have already removed those lines from batch with mem::take (log_push.rs:323-365), so an RPC closure during handoff silently loses them without rebuffering or incrementing the new drop counters. Please return the unsent groups for retry after reconnect, or explicitly record every discarded line before returning.