fix(echotest): trim single newline input

[lowgame]

Reproduction

TrimNewlineEnd([]byte("\n")) currently returns the original newline byte instead of an empty slice. The added regression test fails on the previous implementation with:

Should be empty, but was [10]

Root cause

The bounds guard requires the input length to be greater than one, although indexing the last byte is safe for every non-empty slice. This excludes the valid one-byte newline input.

Solution

Allow the trailing-newline check for any non-empty input and add a focused regression test for a sole newline byte.

Tests

• go test ./echotest -run '^TestTrimNewlineEnd$' -count=1 — passed
• go test ./... — passed
• go vet ./... — passed
• go test -race ./... — passed