Add IM placement codes and fluent option builders (#437)

[mesilov]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Issues	Fix #437
License	MIT

Summary

Adds first-class support for Bitrix24 IM widget placements under the new
Bitrix24\SDK\Services\IM\Placements namespace:

• PlacementLocationCodes — public constants for the four IM placement codes:
  IM_TEXTAREA, IM_SIDEBAR, IM_CONTEXT_MENU, and the deprecated-since-im 25.1600.0
IM_SMILES_SELECTOR.
• PlacementOptionsInterface with build(): array — contract for any object
  that produces the OPTIONS payload for placement.bind.
• AbstractPlacementOptions — shared fluent base providing context(), role(),
  and extranet() setters, backed by the string-backed enums ChatContext, Role,
  and ExtranetAvailability. Multi-value context() joins with ; per API contract.
• TextareaPlacementOptions — required iconName, plus color(), width(), height().
• SidebarPlacementOptions — required iconName, plus color().
• ContextMenuPlacementOptions — only the shared base setters.
• PlacementColor — string-backed enum covering all 17 documented colors.

The Placement::bind() service method now accepts PlacementOptionsInterface|array
for its $options argument. Passing a builder normalizes to build() internally;
existing array callers remain fully compatible — this is a pure type widening,
not a breaking change.

Usage

use Bitrix24\SDK\Services\IM\Placements\ChatContext;
use Bitrix24\SDK\Services\IM\Placements\ExtranetAvailability;
use Bitrix24\SDK\Services\IM\Placements\PlacementColor;
use Bitrix24\SDK\Services\IM\Placements\PlacementLocationCodes;
use Bitrix24\SDK\Services\IM\Placements\Role;
use Bitrix24\SDK\Services\IM\Placements\TextareaPlacementOptions;

$options = (new TextareaPlacementOptions('fa-bug'))
    ->context(ChatContext::User, ChatContext::Chat)
    ->role(Role::User)
    ->color(PlacementColor::Aqua)
    ->width(420)
    ->height(320)
    ->extranet(ExtranetAvailability::Denied);

$serviceBuilder
    ->getPlacementScope()
    ->placement()
    ->bind(
        PlacementLocationCodes::IM_TEXTAREA,
        'https://example.com/widget',
        ['ru' => 'Мой виджет', 'en' => 'My widget'],
        $options,
    );

Quality gate

• make lint-cs-fixer — green
• make lint-rector — green
• make lint-phpstan — green
• make lint-deptrac — green (no new violations)
• make test-unit — 766 tests, 1957 assertions pass

Notes

• All new code lives inside Services layer; depends only on Core transitively — no new
  architectural boundary crossings.
• PlacementLocationCodes intentionally remains a plain class with public const (not an
  enum) so that the deprecated IM_SMILES_SELECTOR constant can carry a @deprecated
  PHPDoc without the friction of enum-case deprecation.
• Unit tests cover each concrete option class: minimal payload, full payload, and fluent
  chaining returns $this.

Closes #437