test(planner): extract repeated literals to satisfy goconst

[bdchatham]

Follow-up to #380. The buildRunningPlan-guard tests added new occurrences of validator-0-key, validator-0-nodekey, and 1000000usei that pushed those literals over goconst's threshold on the new lines, so the lint job went red on main (test / test-integration / verify-generated were green and #380 auto-merged on the required checks).

Fix: hoist the key-secret names to package-level test constants (testSigningKeySecret, testNodeKeySecret), reuse the existing testAccountBalance, and name the RolloutComplete reason in the envtest (reasonRolloutComplete). No behavior change; planner unit tests and the BYO-validator envtest still pass.

[cursor]

PR Summary

Low Risk
Test-only constant extraction and assertion renames; no production code paths touched.

Overview
Fixes goconst failures on main by deduplicating repeated string literals in planner and envtest code—no runtime or test-behavior changes.

In internal/planner/node_update_test.go, adds package constants testSigningKeySecret and testNodeKeySecret for validator secret names used in image-drift planner tests, and uses the existing testAccountBalance for genesis ceremony account balance instead of inline "1000000usei".

In internal/controller/nodedeployment/envtest/inplace_rollout_validator_test.go, introduces reasonRolloutComplete (documented as matching the SND controller’s finished-rollout reason) and asserts against that constant instead of a raw "RolloutComplete" string.

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