_check_binary_document_write stat'ed the controller host only, so a binary
(.sqlite/.pdf/...) that existed solely in the task's execution target
(Docker/SSH/... namespace) was treated as a new file and destroyed by a
plain-text write_file/patch that even reported verified:true (#122662).
The existence decision now goes through one tri-state helper
(_target_regular_file_state: exists/absent/unavailable) that asks the LIVE
file-ops layer - the same backend the write executes on. Host-backed envs
keep today's Path.is_file semantics (OSError -> proceed); other backends are
probed via ShellFileOperations._probe_regular_file, whose missing/not_regular
answers prove absence while every other status fails closed with a retry
message. Locality comes from the live environment object
(_file_ops_uses_host_paths), never env_type strings or class-name hint tables
(VercelSandboxEnvironment is unclassified there). The PDF and generic binary
refusal messages are unchanged verbatim; opaque-document and SQLite-sidecar
unconditional refusals are untouched.
_stale_overwrite_blocker keeps its host-only probe on purpose: remote reads
never record a full_write_baseline (version stability requires host metadata),
so converting it would refuse every remote overwrite of a file the task fully
read. Tracked as follow-up.
(cherry picked from commit 8b5eb26d57d7ef7d1d975de0ef5e1e8abd9a513d)
d17ff29f7d