fix: avoid terminal wait without input

[lowgame]

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features). Not applicable: this is a bug fix with no new API.

Problem

ReleaseTerminal always waits for readLoopDone, even when the program was created with WithInput(nil). No input reader or read loop exists in that configuration, so the wait can only end at the hard-coded 500 ms timeout. This also delays commands run through ExecProcess, which releases the terminal first.

The regression calls ReleaseTerminal on a no-input program and fails on the unmodified ff51ba4c51f85875761b15a64f9ab9aa0eaa9fa1 baseline:

--- FAIL: TestReleaseTerminalWithNilInputDoesNotWait (0.50s)
    exec_test.go:148: ReleaseTerminal took 500.652417ms without an input reader

Change

Wait for the input read loop only in the same branch that has a cancelReader to cancel. Programs with an initialized reader retain the existing cancel-and-wait behavior; no-input programs proceed immediately.

A focused regression test pins the no-input behavior.

Validation

• Baseline reproduction: go test -run '^TestReleaseTerminalWithNilInputDoesNotWait$' -count=1 . — failed as expected after about 500 ms.
• go test -run '^TestReleaseTerminalWithNilInputDoesNotWait$' -count=10 . — passed.
• go test ./... — passed.
• go test -race ./... — passed.
• go vet ./... — passed.
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.9.0 run --timeout=10m — passed with 0 issues.
• (cd examples && go test ./...) — passed; packages without tests compiled successfully.
• git diff --check — passed.