perf: build the `FileMap` of a parser input only once

This PR makes `Lean.Parser.mkInputContext` compute the `FileMap` of its input only once, after normalizing line endings. Its `endPos` and `endPos_valid` parameters, which required computing the `FileMap` of the original input as well, are removed; callers that parse a part of a file with an existing `FileMap` should use `InputContext.mk` with that `FileMap` instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>