Make the tellg failure check explicit and document the EPContext write gate error handling

[GopalakrishnanN]

Description

Two follow-ups from automated review of the EPContext sample helper, both no-functional-change.

1. tellg() failure check. The file-size read held the result in a std::streampos and compared it with 0, which works only through the implicit fpos to streamoff conversion. Holding it in a std::streamoff makes it a plain integral comparison against the documented pos_type(-1) failure value.

2. Documented the error_code handling in EnsureRegularFileForWrite. A reviewer flagged that a failing symlink_status falls through and skips the symlink check, and suggested failing closed. I verified the behavior before changing anything, and did not apply that fix, because it would break the common path:

case	error_code	file_type
missing file in an existing dir (normal new-file write)	set (2)	not_found
missing parent directory	set (3)	not_found
invalid name (a genuine error)	set (123)	not_found
existing regular file	clear	regular

On MSVC a simply-missing target sets error_code, so failing closed on any error would reject ordinary new-file writes. Comparing against std::errc::no_such_file_or_directory does not separate the cases either: ERROR_INVALID_NAME maps to that same condition, so a genuine error is indistinguishable from a benign absent path by either the code or the type.

The residual exposure is small: a target whose type cannot be determined fails at the ofstream open anyway. So the existing fall-through is deliberate, and this adds a short comment recording that rather than changing the guard.

Motivation and Context

This file is sample/reference code EP authors are expected to copy, so a subtly non-obvious guard is worth documenting, and a comparison that relies on an implicit conversion is worth making explicit.

Follow-up to #29294. Raised by automated review on #31195, where the comments landed on a stale diff.

Verified: clang-format clean, within 120 columns, onnxruntime_autoep_test builds, and the EpContextDataUtils tests pass (11 passed, 2 skipped - both symlink tests, which need privileges on Windows).