impl(pubsub): introduce Session

[dbolduc]

Part of the work for #3957

Introduce a Session, which will be a public-facing type. Applications interact with this to stream messages.

The streamed item is a Result<(Message, Handler)>. Applications are forced to handle stream errors, if any.

As of 2026-01-06, this is not reflected in go/cloud-rust:subscriber-api. I need to do an update on that thing. I want to get something, anything working first.

I tried to keep this PR minimal. That means it is missing things like:

1. keepalives
2. lease management
3. graceful shutdown
4. stream retries
5. exactly once messages
6. etc.

I have plans for 1-3. I am sure 4-6 can be done.

[codecov]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.76%. Comparing base (5be1cb5) to head (3fecf55).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pubsub/src/subscriber/session.rs	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4177      +/-   ##
==========================================
+ Coverage   94.74%   94.76%   +0.02%     
==========================================
  Files         186      187       +1     
  Lines        7035     7068      +33     
==========================================
+ Hits         6665     6698      +33     
  Misses        370      370              

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[dbolduc]

PTAL