refactor: split large agent-gateway classes to remove structural suppressions

[jorisjonkers-dev-agents]

Follow-up from lint sweep (PR #10). Split RunnerSessionBinder / HttpAgentGatewayClient / Fabric8AgentRunnerOrchestrator / SessionAttachHandler to remove the documented TooManyFunctions/LargeClass @Suppress, with regression tests. See CANDIDATES-agents-api.md.

[jorisjonkers-dev-agents]

Ready to pick up

Follow-up from the lint sweep (PR #10). Four classes carry @Suppress("TooManyFunctions", "LargeClass") because they exceed detekt thresholds. Each should be split into collaborators with regression coverage.

Current state (verified):

• RunnerSessionBinder.kt — 930 lines, @Suppress("TooManyFunctions", "LargeClass"). Responsibilities: session binding lifecycle, keepalive, transcript relay, runner pod wait logic.
• Fabric8AgentRunnerOrchestrator.kt — 947 lines, @Suppress("TooManyFunctions", "LargeClass"). Responsibilities: pod create/delete, log streaming, readiness checks, node-selector resolution.
• SessionAttachHandler.kt — 641 lines, @file:Suppress("TooManyFunctions", "LargeClass"). Responsibilities: durable attach, live attach, WebSocket input/output, error/close handling.
• HttpAgentGatewayClient.kt — 414 lines, @file:Suppress("LargeClass") + @Suppress("TooManyFunctions"). Responsibilities: headless job, session lifecycle, transcript, health API calls.

Acceptance criteria:

• Each class split into cohesive collaborators, each below detekt's TooManyFunctions (15) and LargeClass (600 lines) thresholds.
• All @Suppress("TooManyFunctions", "LargeClass") annotations removed from these classes.
• Existing integration tests pass; new unit tests cover the extracted collaborators.
• Note: CANDIDATES-agents-api.md (referenced in original issue) does not exist in the current repo; this comment supersedes it.

Files involved:

• api/src/main/kotlin/…/application/sessionbinding/RunnerSessionBinder.kt
• api/src/main/kotlin/…/infrastructure/k8s/Fabric8AgentRunnerOrchestrator.kt
• api/src/main/kotlin/…/infrastructure/ws/SessionAttachHandler.kt
• api/src/main/kotlin/…/infrastructure/integration/HttpAgentGatewayClient.kt