fix(rpc): close WaitGroup Add/Wait race in Engine

[ernado]

Summary

Fixes a data race between Engine.Close and Engine.Do reported by the race detector (seen e.g. in gotd/botapi's TestRunEndToEnd).

Do checked the atomic closed flag and then called e.wg.Add(1) as a separate step, while Close stored the flag and called e.wg.Wait(). Between Do's check and its Add, Close could mark the engine closed and have Wait return on a zero counter, after which Do would call Add(1) — violating the sync.WaitGroup contract that an Add raising the counter from zero must happen-before Wait.

WARNING: DATA RACE
Write at ... by goroutine 248:
  rpc.(*Engine).Close()   engine.go:293   (wg.Wait)
  rpc.(*Engine).ForceClose()
  ...
Previous read at ... by goroutine 264:
  rpc.(*Engine).Do()      engine.go:81    (wg.Add)
  ...

Fix

Guard the close-check and wg.Add(1) together under the existing mux so they cannot interleave with Close:

• closed becomes a mux-guarded bool (was an atomic uint32).
• Do takes mux, returns ErrEngineClosed if closed, else wg.Add(1), then unlocks.
• Close sets closed = true under mux, then wg.Wait() outside the lock (so pending requests can still Done).
• Removes the now-unused isClosed helper.

Testing

go test -race -count=1 ./rpc/... ./mtproto/... passes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.23%. Comparing base (5bd59ff) to head (3ed5cd2).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1776      +/-   ##
==========================================
+ Coverage   71.10%   71.23%   +0.12%     
==========================================
  Files         501      503       +2     
  Lines       23501    23577      +76     
==========================================
+ Hits        16711    16795      +84     
+ Misses       5569     5550      -19     
- Partials     1221     1232      +11     

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