fix(entries): exclude current entry from slug uniqueness on update (#27)

[SAY-5]

Closes #27.

Reproduction (from the issue)

{
  \"action\": \"update\",
  \"collection\": \"your_collection\",
  \"id\": \"<entry-uuid>\",
  \"data\": { \"title\": \"Updated Title\" }
}

fails with:

Field validation failed: slug: This value has already been taken.

even though slug is not part of data.

Root cause

updateEntry() always re-injected the entry's current slug into the FieldsValidator payload:

$mergedData = array_merge($entry->data()->all(), $data);
$mergedData['slug'] = $entry->slug();   // <-- always added

UniqueEntryValue then ran against that slug without an exclusion, so Statamic found the slug on the entry being updated and rejected it as "already taken". Every update through the MCP tool failed.

Fix

Mirror the slug handling already used in createEntry(), with one small addition: pass $entry->id() as the $except argument so UniqueEntryValue excludes the current entry from the uniqueness check. Slug-rename, idempotent-slug, and slug-not-supplied paths are now all handled before blueprint validation, and the line that re-injected the slug into mergedData is removed so FieldsValidator never sees the slug column.

if (array_key_exists('slug', $data)) {
    $newSlug = is_string($data['slug']) ? $data['slug'] : '';
    $slugValidator = Validator::make(['slug' => $newSlug], [
        'slug' => [
            'required',
            'string',
            new UniqueEntryValue($entry->collectionHandle(), $entry->id(), $site),
        ],
    ]);
    if ($slugValidator->fails()) { ... return error ... }
    $entry->slug($newSlug);
    unset($data['slug']);
}

Test plan

Added four regression tests in tests/Feature/Routers/EntriesRouterTest.php:

• test_update_entry_without_slug_in_data_succeeds — exact repro from Update action fails with "slug: This value has already been taken" for existing entries #27 (no slug field): update now succeeds.
• test_update_entry_with_unchanged_slug_succeeds — resending the current slug is idempotent.
• test_update_entry_with_new_unique_slug_succeeds — renaming to a unique slug works and is persisted.
• test_update_entry_to_existing_slug_returns_error — colliding with another entry's slug still surfaces the validation error (the legitimate case the original check was protecting against).

Results:

• vendor/bin/pest tests/Feature/Routers/EntriesRouterTest.php → 18 passed; 0 failed (4 new + 14 pre-existing).
• vendor/bin/pest --filter test_update_entry → 18 passed; 0 failed across feature + integration suites that exercise the update flow (DateNormalizationTest, GroupFieldBugTest, TermsFieldUpdateTest, UpdateEntryBugTest).

Notes

• The proposed fix in the issue is implemented as-is.
• No public API changes; only the internal validation flow.