ci: extend scenario AWS session

[bill-ph]

Summary

• request a 16,200-second OIDC role session for scenario-dev, matching its 270-minute job timeout
• document the cross-repository IAM maximum-session dependency
• pin the requested duration in the scenario workflow test

Why

Long dbt scenarios can outlive the action's default one-hour AWS session. The scenario keeps running in-cluster, but subsequent Kubernetes calls then cannot collect artifacts, run diagnostics, or tear down the isolated namespace.

Dependency

Depends on PostHog/posthog-cloud-infra#9457 being merged and applied first. The workflow's role assumption will fail if it requests 16,200 seconds while the deployed role maximum remains 3,600 seconds.

Validation

• red: just test-scenario failed because role-duration-seconds: 16200 was absent
• green: just test-scenario
• just lint

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	0	2	0
E2E/journey files	0	0	0
Workflow files	0	1	0

Signals

• Test cases: +0 / -0
• Assertions: +0 / -0
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: neutral or increased

No coverage-reduction warnings detected.