refactor: break message verification handling out into free function

[davidsteiner]

In the continued effort to rip apart the monolithic session code, this PR splits up message verification issues into two variants:

• sequence gaps which need to be handled by the state, so handling can't easily be centralised in a free function
• actual errors, which are handled the same way regardless of state

This has allowed handle_verification_error to be moved into the inbound module as a free function and it no longer handles sequence gaps.

[codecov]

Codecov Report

❌ Patch coverage is 93.70861% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.06%. Comparing base (1aaf9d8) to head (aa5cb67).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/hotfix/src/session/state/awaiting_resend.rs	85.10%	7 Missing ⚠️
crates/hotfix/src/session.rs	87.50%	6 Missing ⚠️
crates/hotfix/src/session/state/awaiting_logout.rs	80.00%	4 Missing ⚠️
crates/hotfix/src/session/state.rs	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
+ Coverage   90.95%   91.06%   +0.10%     
==========================================
  Files          92       94       +2     
  Lines        9157     9232      +75     
==========================================
+ Hits         8329     8407      +78     
+ Misses        828      825       -3     

Flag	Coverage Δ
core	90.94% <93.70%> (+0.10%)	⬆️

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