fix: Take a read lock rather than an exclusive lock in getContextForTx

[SillyZir]

Description

Ref: #24634 (partial — see Scope)

getContextForTx acquired app.mu exclusively, but reads only:

• app.stateManager.GetState(mode) — the state manager is independently
  synchronised by its own sync.RWMutex (baseapp/state/manager.go)
• app.sigverifyTx
• app.GetConsensusParams(ctx), which does not touch app.mu

app.mu documents itself as protecting name, version and appVersion.
Every other use in baseapp respects that scope — the readers (Name,
Version, AppVersion, Info) take RLock, and the only writers are the
setup-time setters in options.go. getContextForTx was the sole exception.

Since it is called per transaction from runTx, that exclusive acquisition
serialised transaction execution against those readers for no reason.

Scope

This does not address the stateManager.stateMut contention #24634 also
implies. CreateQueryContextWithCheckHeader does not take app.mu on its hot
path, so query latency specifically is unaffected except via Name() on the
"not ready" branch. Filed as a partial improvement rather than a fix — happy to
close it if you would rather see the issue addressed as a whole.

Testing

go test -race ./baseapp/... passes (baseapp 185s, plus blockexec, oe,
txnrunner). No regression test: the change narrows a lock scope without
altering behaviour, so no input fails before and passes after.

[greptile-apps]

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