fix(discord): cap forum thread names by UTF-16 units, not code points

[briandevans]

This is a sibling follow-up to #60113

• What fix(discord): bound component labels by UTF-16 units #60113 covered: bounded Discord component/button labels by UTF-16 code units (the field Discord actually validates), adding test_truncates_emoji_choice_label_by_utf16_limit.
• What fix(discord): bound component labels by UTF-16 units #60113 did NOT touch: the forum thread-name derivation path, _derive_forum_thread_name, which still capped by Python code points.
• What this adds: migrates that one remaining [:100] code-point slice to the same _prefix_within_utf16_limit helper the adapter already imports, plus a regression test.

What does this PR do?

_derive_forum_thread_name in plugins/platforms/discord/adapter.py caps the
derived Discord forum thread name with first_line[:100] — a code-point slice —
but Discord validates the thread name field in UTF-16 code units (100 max).

A message whose first line opens with emoji / non-BMP text (agents commonly lead
with an emoji header) can exceed 100 UTF-16 units while passing the code-point
cap, so create_thread(name=...) fails with error 50035 ("Must be 100 or fewer
in length") and the entire forum reply is dropped (SendResult(success=False)),
not merely clipped. This is the sibling the UTF-16 hardening in #60113 (component
labels) missed; the adapter already imports _prefix_within_utf16_limit/utf16_len
for exactly this purpose.

Related Issue

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• plugins/platforms/discord/adapter.py: _derive_forum_thread_name now returns
  _prefix_within_utf16_limit(first_line, 100) instead of first_line[:100]
  (and the docstring reflects the UTF-16 unit limit).
• tests/gateway/test_discord_clarify_buttons.py: add
  test_forum_thread_name_capped_by_utf16_units.

How to Test

1. _derive_forum_thread_name("\U0001F600" * 100) — 100 emoji is 200 UTF-16
   units under the old code-point cap; after this change the result is clamped
   to <= 100 units without splitting a surrogate pair.
2. ASCII behavior is unchanged: 200 x still clamps to exactly 100 units.
3. pytest tests/gateway/test_discord_clarify_buttons.py tests/gateway/test_discord_send.py -q → 40 passed. The new test fails before the fix (200 > 100), passes after.

Follow-up (out of scope here): the Choice.name and describe-hint [:100] caps
in the same adapter are the remaining code-point siblings and can be migrated
separately.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/gateway/test_discord_clarify_buttons.py tests/gateway/test_discord_send.py -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 15 (Darwin 25.4)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — updated the _derive_forum_thread_name docstring
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact (Windows, macOS) — pure string logic, platform-independent

[Copilot]

Pull request overview

This PR fixes a Discord forum-thread posting edge case where derived thread names could exceed Discord’s 100 UTF-16 code unit limit when the first line contains non-BMP characters (e.g., emoji), causing create_thread(name=...) to fail and drop the forum reply. It aligns forum thread-name truncation with the existing UTF-16-safe truncation approach already used elsewhere in the Discord adapter.

Changes:

• Update _derive_forum_thread_name to cap by UTF-16 code units via _prefix_within_utf16_limit(..., 100) instead of slicing by Python code points ([:100]).
• Add a regression test covering emoji-heavy and ASCII-heavy inputs to ensure derived forum thread names fit Discord’s UTF-16 budget.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
plugins/platforms/discord/adapter.py	Switch forum thread-name truncation to UTF-16-unit-safe prefixing and update the docstring to reflect Discord’s actual validation unit.
tests/gateway/test_discord_clarify_buttons.py	Add regression coverage ensuring forum thread-name derivation stays within the 100 UTF-16 code unit limit (emoji + ASCII cases).

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[teknium1]

Thanks for isolating the forum-thread path. The proposed replacement of the current first_line[:100] at plugins/platforms/discord/adapter.py:7794 with the existing UTF-16 helper is correct: that value is used by both forum creation paths (:2119 and :7975), and _prefix_within_utf16_limit enforces the unit budget in gateway/platforms/base.py:156-172.

Problems

• The same unit mismatch remains in auto-thread naming: plugins/platforms/discord/adapter.py:5332-5335 caps by code points before the names reach create_thread at :5355 and :5367. The handoff path has another 80-code-point cap at :5495, passed to creation at :5503 and :5521. An 80-emoji input exceeds an 80-unit budget.

Suggested changes

• Apply the same UTF-16-safe cap to those two thread-name producers and add emoji coverage next to tests/gateway/test_discord_slash_commands.py:639.

Automated hermes-sweeper review.