[WIP] Implement GraphQL schema completion and subscription support

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Original prompt

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This section details on the original issue you should resolve

<issue_title>GraphQL Schema Completion and Subscription Support</issue_title>
<issue_description>### Context

This issue implements the roadmap item 'GraphQL Schema Completion and Subscription Support' for the api domain. It is sourced from the consolidated roadmap under 🟠 High Priority — Near-term (v1.5.0 – v1.8.0) and targets milestone v1.7.0.

Primary detail section: GraphQL Schema Completion and Subscription Support

Goal

Deliver the scoped changes for GraphQL Schema Completion and Subscription Support in src/api/ and complete the linked detail section in a release-ready state for v1.7.0.

Detailed Scope

GraphQL Schema Completion and Subscription Support

Priority: High
Target Version: v1.7.0

graphql.cpp implements a full parser and query executor but lacks mutation resolvers, schema introspection (__schema, __type), and subscription over WebSocket. Complete the schema to cover documents, graph edges, vector search, and geospatial queries; add subscription operation support backed by cdc::Changefeed.

Implementation Notes:

• [x] Add SchemaRegistry class to graphql.cpp; auto-build from registered TypeDefinition objects at server start.
• [x] Implement __schema and __type introspection resolvers; required by all major GraphQL clients (Apollo, Relay).
• [x] Subscription transport: use Boost.Beast WebSocket upgrades; create graphql_ws_handler.cpp implementing the graphql-transport-ws protocol (not the legacy subscriptions-transport-ws).
• [x] Wire cdc::Changefeed::subscribe(filter) as the event source for subscription { onChange(collection: "...") { ... } }. Implemented: Changefeed::subscribe(SubscriptionFilter, SubscriptionCallback) + SubscriptionHandle RAII type in changefeed.h/cpp; wired in GraphQLWsHandler::handleSubscribe() via extractOnChangeCollection().
• [x] Enforce QueryLimits::maxSubscriptions per connection to prevent fan-out DoS.
• [ ] In graphql.h, the Parser class explicitly documents "Not yet supported: Fragments, Directives, Inline fragments." Implement parseFragmentDefinition() and parseInlineFragment() in graphql.cpp — without fragment support, clients using Apollo's automatic persisted query fragments or any relay-style fragment composition will fail at parse time.
• [ ] graphql.h::Parser::error() is documented as deprecated ("Deprecated: Use Result<T> return types instead of error() method") but the method still exists in the class definition. Remove it after migrating all call sites in graphql.cpp to return themis::Result<T> with structured ParseError objects to eliminate the dual error-reporting path.
• [ ] Schema::introspect() in graphql.cpp only handles __schema and __type fields. The GraphQL June 2018 spec also requires __typename on every composite type, __Field, __InputValue, __EnumValue, and __Directive meta-types. Add these to Schema::introspect() so introspection-based tooling (code generators, schema diffing tools) works fully.
• [ ] Executor::executeSelections() in graphql.cpp resolves fields serially in a range-for loop. For independent sibling fields that each invoke storage I/O, this means sequential round-trips. Add parallel field resolution via std::async or a small task graph; guard behind a QueryLimits::parallel_fields_enabled flag to allow gradual rollout.

Performance Targets:

• GraphQL parse + validate + execute for a 10-field document query in < 2 ms (p99) under 500 concurrent HTTP/2 connections.
• Subscription event delivery latency < 50 ms from Changefeed event emission to WebSocket frame sent.

API Sketch:

# New subscription type (graphql.cpp SchemaRegistry)
type Subscription {
  onChange(collection: String!, filter: ChangeFilter): ChangeEvent!
}

type ChangeEvent {
  sequence: Int!
  type: ChangeType!
  key: String!
  document: JSON
  timestampMs: Int!
}

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Acceptance Criteria

• Add SchemaRegistry class to graphql.cpp; auto-build from registered TypeDefinition objects at server start.
• Implement __schema and __type introspection resolvers; required by all major GraphQL clients (Apollo, Relay).
• Subscription transport: use Boost.Beast WebSocket upgrades; create graphql_ws_handler.cpp implementing the graphql-transport-ws protocol (not the legacy subscriptions-transport-ws).
• Wire cdc::Changefeed::subscribe(filter) as the event source for subscription { onChange(collection: "...") { ... } }. Implemented: Changefeed::subscribe(SubscriptionFilter, SubscriptionCallback) + SubscriptionHandle RAII type in changefeed.h/cpp; wired in GraphQLWsHandler::handleSubscribe() via extractOnChangeCollection().
• Enforce QueryLimits::maxSubscriptions per connection to prevent fan-out DoS.
• In graphql.h, the Parser c...

• Fixes GraphQL Schema Completion and Subscription Support #3876

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