fix(types): widen canceled Literal[True]=True to Literal[True]|None=None on request types

[bokelley]

Closes #641

canceled: Literal[True] = True on UpdateMediaBuyRequest and PackageUpdate was a datamodel-codegen misfire from "const": true boolean schema properties. Any buyer who omitted the field — e.g. a routine budget or pacing update — silently triggered irreversible media buy cancellation via the Pydantic default. This PR fixes all four request-type emissions by adding fix_canceled_literal_defaults() to scripts/post_generate_fixes.py, which rewrites the annotation to Literal[True] | None and the default to = None. Wire semantics are preserved: the field still only accepts true when present, and canceled=False remains a Pydantic ValidationError. Response-side canceled: bool | None = False status indicators are out of scope — their defaults are already non-destructive.

What's in the diff

File	Change
scripts/post_generate_fixes.py	New _CANCELED_FIELD_RE regex + fix_canceled_literal_defaults() wired into main(). Covers 3 target files (4 total emissions). Idempotent: `Literal[True]
src/adcp/types/generated_poc/media_buy/update_media_buy_request.py	UpdateMediaBuyRequest.canceled: Literal[True] = True → `Literal[True]
src/adcp/types/generated_poc/media_buy/package_update.py	PackageUpdate.canceled: same change
src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_request.py	Both inline classes: same change (2 occurrences)
tests/test_canceled_literal_default.py	11 behavioral tests: default-to-None, explicit True accepted, wire payload absent/present, False rejected, regex idempotency

What was tested

• pytest tests/ -q: 4316 passed, 30 skipped (1 pre-existing network failure test_real_tls_handshake_still_validates_hostname also fails on main)
• mypy src/adcp/types/generated_poc/media_buy/update_media_buy_request.py src/adcp/types/generated_poc/media_buy/package_update.py: Success, no issues
• ruff check src/: All checks passed

Nits (not fixed — out of PR scope)

• inject_literal_discriminator_defaults() already skips bool-valued Literal[True] (has if not isinstance(value, str): continue), so ordering against it is not load-bearing
• _CANCELED_FIELD_RE hardcodes 4-space indentation — consistent with every other fix in this script; idempotency test catches silent no-ops
• No bundled-type behavioral test — consistent with rest of test suite's approach to bundled types
• dx-expert noted: field description could be updated to explicitly document the sentinel pattern ("Set to true to cancel; omit or None to leave unchanged") — considered a follow-up

Pre-PR review

• code-reviewer: approved — regex matches exactly 4 emissions; idempotency holds; no blockers
• dx-expert: approved — Literal[True] | None = None correct; if req.canceled: still falsy-safe for None; no migration doc needed; unused import re nit fixed before PR

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/cse_013iJiiNZi9aBGFawGKUNiGw

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