session: add new session package

[Jorropo]

This replaces #563 and #561.

TODO:

• Tests
• Wire bitswap to unwrap the session
• Revert 22fa8b1 and wire the gateway to use this new package.
• Changelog (@Jorropo will do later)
• ¿ tracing ?

Questions:

• Right now consumer is a simple comparable to ensure it doesn't panic the map, it might be interesting to enforce something like interface[S any]{ comparable; SessionBadge() S } because this would enforce at compile time that the session consumer is not mixing different session types together (which would not work).
• Should GetOrCreate return (S, error) instead of S, bool ?
  Passing errors can always be done efficiently through closures as create does not leak, the important part is to not save in the map failed sessions.
• Should it be ContextWithSession(context.Context, ...Option) ? We might want to add something like a request root in there.

[Jorropo]

We ultimately went that route because it satisfy both:

• Session producers don't need a pointer to some already shared object (like blockservice) to inject a session in their context.
• It permits session consumers which don't need to maintain more state than in the session object to only inject their session in the context instead maintaining their own uint64 → session structures for it. This allows such state contained sessions to be cleaned up by the GC automatically.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.29%. Comparing base (4c3a1f2) to head (fa52546).
⚠️ Report is 735 commits behind head on main.

@@            Coverage Diff             @@
##             main     #570      +/-   ##
==========================================
- Coverage   65.58%   65.29%   -0.30%     
==========================================
  Files         206      203       -3     
  Lines       25587    25516      -71     
==========================================
- Hits        16781    16660     -121     
- Misses       7335     7372      +37     
- Partials     1471     1484      +13     

see 15 files with indirect coverage changes

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[lidel]

Closing as stale - this has been open for 2 years with incomplete TODOs and no activity since Aug 2025.

The approach here (context-embedded session registry with GC cleanup) seemed sound and was agreed upon in #567. If someone wants to pick this up, the design discussion in #567 may be still
relevant.

For now, we have the partial fix from #549 (NewSessionContext/EmbedSessionInContext). The nopfs incompatibility (ipfs-shipyard/nopfs#34) remains, but would need this or a similar solution to fully fix.

Happy to reopen (as a new PR) if there's renewed interest in completing this.