fix: allow dag import of 1MiB chunks wrapped in dag-pb

[lidel]

This is mostly a bunch of tests + small fix for dag import.

IPIP-499's (#11148) unixfs-v1-2025 profile uses 1MiB chunks. with --raw-leaves=false, protobuf wrapping pushes blocks slightly over 1MiB. the previous 1MiB SoftBlockLimit rejected these blocks on dag import.

This PR raises SoftBlockLimit to 2MiB to match the bitswap spec, which requires implementations to support blocks up to 2MiB.

• raise SoftBlockLimit to 2MiB per the bitswap spec
  • this is soft limit in CLI when someone tries to import with chunk size bigger than the limit. block put etc errors. user can override and force with --allow-big-block
  • update error messages and help text
• bump boxo (raised ChunkSizeLimit/BlockSizeLimit, 256-byte overhead budget)
• update sharness tests for 2MiB boundary
• add test/cli boundary tests for block put, dag put, dag import, ipfs add (raw and dag-pb wrapped leaves), and bitswap exchange including regression tests for the libp2p message size hard limit (4MiB minus size of CID-related overhead)

• Depends on fix: raise block size limits from 1MiB to 2MiB boxo#1101 landing first
• CC Implement modern CID profile from IPIP-499 #11071
• CC IPIP-499: UnixFS CID Profiles specs#499

[lidel]

Merging when CI finishes to unblock 0.40.0-rc1.

This is a simple fix with low risk: existing users are not affected, we are not changing any defaults, and if someone starts producing data with >1MiB && <2MiB chunks old software is able to fetch 2MiB blocks over bitswap just fine (real limit is slightly below 4MiB).

Most of the diff is regression tests we were missing: boundary tests for block put, dag put, dag import, ipfs add (raw and wrapped leaves), bitswap exchange between two nodes, and regression tests at the libp2p message size hard limit (~4MiB), so we catch regressions in case go-libp2p ships a breaking change in the future.