fix: discover mounted Trino authentication provider paths

[bill-ph]

Summary

Trino authentication readiness assumed both provider files lived under /etc/trino. Coordinators using /etc/trino/coordinator therefore failed readiness even when their authentication files were correctly mounted.

Resolve each provider file from the selected coordinator container's explicit ConfigMap subPath mount. Reject missing or ambiguous mounts, retain credential verification and cache-refresh waiting, and identify the provider in sanitized observation errors. No new deployment setting is required. The runbook documents the supported mount contract.

Validation

• Confirmed the new regression fails before the fix for the coordinator-directory layout and passes afterward.
• just test-trino passed, including both layouts and rejected mount configurations.
• Independent review found no blocking correctness, overengineering, or scope findings.
• just lint reports the same 6 diagnostics as clean main; Kubernetes-tagged lint reports the same 61 diagnostics as clean main. Both baseline comparisons are identical, with no new diagnostics.
• Not yet deployed; live tenant readiness remains to be verified after rollout.

[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	1	0
E2E/journey files	0	0	0
Workflow files	0	0	0

Signals

• Test cases: +1 / -0
• Assertions: +10 / -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.