feat(consensus): make validator execution concurrency configurable (v0.123)

[MuncleUscles]

Refs #1721 — port of the same change targeting v0.121.

What

• Replace the hardcoded asyncio.Semaphore(8) in CommittingState.handle with a module-level VALIDATOR_MAX_CONCURRENT read from the CONSENSUS_VALIDATOR_MAX_CONCURRENT env var (default 8, clamped to >= 1).
• Update the stale comment in backend/protocol_rpc/endpoints.py that referenced the hardcoded value.
• Document the new variable in .env.example.

Why

A 47-validator committee currently runs through a fixed window of 8 concurrent validator executions (~6 sequential rounds), dominating transaction latency. The cap itself is legitimate (GenVM subprocess memory / fd / DB-pool bounds) but the value 8 was arbitrary and not tunable. Deployments with adequate memory can now size it to their committee sizes. Default behavior is unchanged.

Testing done

• python3 -m py_compile on both edited modules.
• No behavior change with the variable unset (default 8).

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[sonarqubecloud]

Quality Gate failed

Failed conditions
25.0% Coverage on New Code (required ≥ 80%)
C Security Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[MuncleUscles]

/run-e2e studio

[MuncleUscles]

/run-e2e studio

[MuncleUscles]

/run-e2e studio

[MuncleUscles]

/run-e2e studio

[MuncleUscles]

/run-e2e studio

[MuncleUscles]

Retargeted to v0.123-dev (rebased, same change). The prior E2E failures were not this PR: the v0.6 e2e train tests the fee-enabled stack (sim_getFeeConfig via genlayer-js v2 / cli v0.40 / wallet v0.2), which exists on v0.123-dev (fee accounting restored in 9e7bd54) but not on the stale v0.123 release branch — so all tooling components died on Method not found: sim_getFeeConfig regardless of this diff. Per the release flow, v0.123 only receives promotions from -dev and shouldn't be a direct PR target.

/run-e2e studio

[MuncleUscles]

/run-e2e studio