feat: Agent graph support

[mattrmc1]

Summary

Adds agent graph support — flag evaluation, graph validation, BFS traversal, graph-level tracking, and resumption tokens. Callers fetch a graph definition via agentGraph(graphKey, context, variables), inspect or traverse the node topology, and track graph-level metrics (invocation success/failure, duration, tokens, path) plus edge-level events (redirect, handoff) through AIGraphTracker.

New types

GraphEdge — immutable edge holding target key and optional handoff metadata map (unmodifiable).

AgentGraphNode — wraps a node key, its resolved AIAgentConfig, and outgoing GraphEdge list. isTerminal() returns true when edges are empty.

AgentGraphFlagValue (internal) — parses the graph flag JSON protocol: root, edges adjacency map, and _ldMeta (enabled, variationKey, version). Defensively handles malformed input without throwing.

AgentGraphDefinition — the resolved graph. Key API: isEnabled(), rootNode(), getNode(key), getChildNodes(key), getParentNodes(key), terminalNodes(), createTracker(). BFS traversal via traverse(fn, ctx) (root-to-leaves) and reverseTraverse(fn, ctx) (terminals-to-root, root always processed last). Both are cycle-safe. Node building and key collection via static helpers.

AIGraphTracker — graph-level at-most-once tracking (trackInvocationSuccess/trackInvocationFailure sharing one guard, trackDuration, trackTotalTokens, trackPath) and multi-fire edge-level events (trackRedirect, trackHandoffSuccess, trackHandoffFailure). Uses AtomicReference.compareAndSet(null, value) for at-most-once. Empty token usage doesn't burn the slot. Resumption token via getResumptionToken() / fromResumptionToken(token, client, context) with version clamping.

AIGraphMetricSummary — immutable snapshot of graph tracker state (success, durationMs, tokens, path, resumptionToken). All nullable except resumptionToken.

Client methods

agentGraph(graphKey, context, variables) on LDAIClient — evaluates the graph flag, validates (enabled -> root present -> all nodes reachable from root -> all child configs enabled), fetches each node's AIAgentConfig passing graphKey for tracker correlation. Returns disabled definition on any validation failure. Emits $ld:ai:usage:agent-graph usage event.

createGraphTracker(resumptionToken, context) — reconstructs an AIGraphTracker from a resumption token.

Other changes

• NoOpAIConfigTracker deleted — no longer needed since all configs get real trackers.
• ResumptionTokens extended with encodeGraph/decodeGraph for graph-specific tokens (fields: runId, graphKey, variationKey, version). Made public for access from AIGraphTracker.
• LDAIConfigTrackerImpl updates: null-arg logs downgraded warn -> debug, trackJudgeResult guards blank metricKey and non-finite score, trackMetricsOf stops clock before extractor (duration tracked even if extractor throws).
• trackerFactory default version changed from 0 to 1.
• agentConfigs() reordered to emit usage count before fetching configs.
• Internal evaluate/buildConfig/buildConfigFromDefault gain graphKey parameter for node-level tracker correlation.

Test plan

• ./gradlew :lib:sdk:server-ai:test passes
• AIGraphTrackerTest (480 lines): invocation success/failure + shared guard, duration, total tokens (including zero-usage skip), path, redirect/handoff multi-fire, base data correctness, variationKey omission, getSummary, resumption token round-trip, concurrency (20-thread contention for invocation and duration)
• AgentGraphDefinitionTest (445 lines): buildNodes, collectAllKeys, traverse/reverseTraverse (including cycles, single-node, diamond), rootNode/getNode/getChildNodes/getParentNodes/terminalNodes, disabled graph behavior, createTracker
• LDAIClientImplTest additions (192 lines): agentGraph usage event, enabled/disabled graph, unreachable node validation, non-enabled child config validation, graphKey threading to child trackers, createGraphTracker delegation
• AgentGraphFlagValueTest (276 lines): parse root/edges/meta, missing fields, disabled flag, malformed input, handoff metadata, edge with missing key skipped
• ResumptionTokensTest additions: graph token encode/decode round-trips
• LDAIConfigTrackerImplTest additions: trackMetricsOf extractor-failure duration tracking, null AIMetrics guard