…channel override keys in migrations

- Add strum EnumIter + IntoStaticStr derives to ReleaseChannel
- Add SupportedPlatform enum with EnumIter
- Make gpui and semver optional deps behind a 'gpui' feature
- Replace hardcoded PLATFORM_AND_CHANNEL_KEYS in migrations with
  ReleaseChannel::iter() and SupportedPlatform::iter()
- Remove test case that expected non-object edit_predictions to skip

…ined

Add test cases to test_move_edit_prediction_provider_to_edit_predictions and
test_migrate_experimental_sweep_mercury that verify migrations work correctly
when the same setting appears simultaneously at the root level, inside a
platform override (e.g. macos/linux), and inside a profile.

Update the doc comment to make it clear the returned values are the
JSON keys used in settings files for platform-specific overrides.

The branch had changed .log_err() to .ok() and replaced
expr.log_err() with let _ = expr, which silently discards errors.
Per project rules, errors should never be silently discarded.
Restore all affected call sites to use .log_err() and re-add the
util::ResultExt import.

…rplate

Both m_2026_02_02 and m_2026_02_03 had identical code for iterating over
the root settings object, platform/channel overrides, and profile
settings. Extract this into a shared migrate_nested_settings() helper in
the migrations module that takes a callback and applies it to each
nested settings map.

…migrations

Refactor make_file_finder_include_ignored_an_enum and
make_relative_line_numbers_an_enum to use the migrate_nested_settings
helper so they correctly handle platform, release-channel, and profile
override objects.

Each migration now extracts its core logic into a migrate_one helper
that operates on a single Map, and delegates to migrate_nested_settings
to apply it across all nested scopes.

Added tests for platform overrides and profile settings for both
migrations.

…s-migrations

Replace the release_channel dependency (which required feature-gating
gpui and touching 40+ Cargo.toml files) with a simple inlined constant
of the override keys used in settings JSON.

…oded strings

Feature-gate gpui/semver as optional in release_channel (default on),
add SupportedPlatform enum and strum::EnumIter derives. The migrator
uses a direct path dep with default-features = false to avoid pulling
in gpui, so no other crate's Cargo.toml needs to change.

Also updates m_2025_11_25::remove_context_server_source to use
migrate_nested_settings so context_servers.*.source is removed from
platform, release-channel, and profile overrides too.

Add migrate_nested_language_setting helper that composes both nesting
strategies: for each level (root, platform overrides, release-channel
overrides, profiles), it applies the migration to the value itself and
to each language under its 'languages' key.

Update m_2025_10_01, m_2025_10_02, and m_2025_10_16 to use it. Remove
the now-unused migrate_language_setting.

Extract platform and release-channel override fields from
UserSettingsContent into PlatformOverrides and ReleaseChannelOverrides
structs with #[serde(flatten)]. Each struct exposes an OVERRIDE_KEYS
constant listing its field names.

The migrator imports these constants from settings_content directly,
so the override keys used in migrations are always in sync with the
actual settings struct fields. This removes the release_channel and
strum dependencies from the migrator, and reverts release_channel
back to its original state (no feature gating needed).

Add a settings_overrides! macro in settings_content that generates
the struct definition, OVERRIDE_KEYS const, and get_by_key accessor
from a single field list. The field names appear exactly once.

for_os() and for_release_channel() now use get_by_key() instead of
hand-written match arms with duplicated key strings.