[codex] unify apply patch parsing

[pakrym-oai]

Why

apply_patch maintained separate batch and streaming parsers for the same patch grammar. That duplicated the parsing rules and allowed final execution to disagree with the live streamed preview.

What changed

• Make StreamingPatchParser the single owner of hunk and environment ID parsing.
• Keep heredoc and outer patch-boundary normalization in the existing parse_patch wrapper, preserving its public API.
• Reject non-whitespace content after *** End Patch and preserve separator handling after *** End of File.
• Reject duplicate environment ID preambles explicitly.
• Remove the duplicate batch hunk parser and its implementation-specific tests.

The change removes 201 net lines while retaining focused coverage for the unified parser's boundary behavior.

Validation

• just test -p codex-apply-patch
• Compared a 24-hour corpus of 2,788,059 observed apply_patch payloads against the previous batch parser. All 2,779,502 accepted payloads produced identical hunks, canonical patch text, and environment IDs; the remaining 8,557 payloads were rejected by both parsers, with zero acceptance or payload mismatches.