refactor(mcp): converge OAuth integration ownership

[limityan]

Summary

• move MCP OAuth credential storage and authorization bootstrap behind the Services integration owner
• keep Core's public compatibility facade plus product callback/session/reconnect orchestration
• route Desktop OAuth credential status through the assembled MCP manager instead of the Core global facade
• split the product-owned callback page/HTTP adapter into a private manager module without changing its behavior

Architecture boundary

Concern	Owner after this PR
RMCP SDK, credential vault/store, authorization bootstrap	Services integrations
Product callback page, OAuth session/cancel/reconnect orchestration	Core product assembly
Desktop command projection	Desktop via assembled Core manager

Core's direct rmcp manifest edge changes from 1 to 0, and product-full no longer selects dep:rmcp directly. Services integrations remains the single direct RMCP owner. RMCP is still transitively present in full MCP products; this PR establishes ownership and does not claim to remove the full cold-build graph.

Compatibility and scope

• preserves the existing Core OAuth facade for callers
• preserves callback locale selection, HTML escaping, first-callback delivery, timeout, cancellation, token exchange, and reconnect flow
• adds an existing-boundary guard preventing new direct RMCP manifest edges outside Services integrations
• adds one owner-local manager regression test in the existing Core test target
• does not add or change GitHub Actions jobs, workflows, test targets, or product delivery features

Dependency impact

Metric	Before	After
Core direct rmcp dependency	1	0
Core product-full direct dep:rmcp selection	1	0
Services integrations direct rmcp owner	1	1

Validation

• pnpm run check:core-boundaries:test (36/36)
• pnpm run check:core-boundaries
• cargo test -p bitfun-services-integrations --no-default-features --features mcp --test mcp_contracts (42/42)
• cargo check -p bitfun-core --no-default-features
• cargo test -p bitfun-core service::mcp::server::manager::auth_callback::tests::callback_page_prefers_the_product_locale_and_escapes_provider_errors -- --exact
• cargo test -p bitfun-core service::mcp::server::manager::tests::oauth_credentials_follow_the_manager_injected_data_dir -- --exact
• cargo check -p bitfun-desktop