perf: keep sse.Register and convenience generic parts small

[wolveix]

Follow-up to #1115. Go compiles a separate copy of a generic function body for each distinct type layout, so the remaining per-operation generics still duplicated code that never touches their type parameters.

• sse.Register[I]: schema building moved into addEventStreamResponse, response streaming into streamResponse, and stream is no longer generic (it takes a run(ctx, send) closure). Only the huma.Register call and the closure over input/f remain generic.
• convenience[I, O] (behind huma.Get, Post, etc.): operation building moved into convenienceOperation. The typed nil *O is still passed through for the ID/summary generators and _convenience_*_out metadata used by groups.

No public API or behaviour changes.

Numbers

Probe program with 20 huma.Get + 20 sse.Register operations, each with distinct types:

	main	this PR
generic code per op pair	9,104 B	2,384 B
binary size	8,394,162 B	8,190,306 B

Roughly 5.7 KB saved per SSE operation and 1 KB per convenience operation. The remainder is mostly the Register shim from #1115.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.18%. Comparing base (e4a9e0c) to head (2a6a498).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1117      +/-   ##
==========================================
+ Coverage   93.17%   93.18%   +0.01%     
==========================================
  Files          24       24              
  Lines        4997     5005       +8     
==========================================
+ Hits         4656     4664       +8     
  Misses        273      273              
  Partials       68       68              

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