[chain-pr 6/12] Rewrite SessionManager with async start, deterministic sampling, and Promise-based init

[johnHackworth]

This PR is part of a chain split from #0 v7 by chain-pr.

Changes

New unified SessionManager built on the reactive store strategy, returning a Promise. Adds findTrackedSession with sample-rate gating, TRACKED_SESSION_MAX_AGE cap, expireDate cap at SESSION_TIME_OUT_DELAY, cross-tab adoption guard, telemetry init metric, and SessionRenewalEvent debug context. Adds collectAsyncCalls and createSessionManagerMock test helpers, plus trackingConsent.onGrantedOnce.

Refactoring rationale

Replaces the per-product (RUM/Logs) session managers with a single async SessionManager that returns a Promise and supports deterministic sampling, isReplayForced via session state, and a TRACKED_SESSION_MAX_AGE cap. The signature change from synchronous to Promise-based propagates through every consumer, which is why the entire startRum/startLogs/preStart pipeline must follow in the next group. Also extends TrackingConsent with onGrantedOnce to drive deferred initialization.

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Draft — pending author review. Merge in order unless marked parallel.

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