feat(tests): derive splits from Depot CI matrix

[ischolten]

Summary

• derive omitted --index and --total values from Depot CI matrix context
• preserve explicit flag precedence and explicit --total 1 passthrough behavior
• validate matrix-derived values while retaining unsplit tests run behavior outside split mode

Verification

• go test ./...
• Depot CI matrix validation

---

Note

Medium Risk
Changes when test splitting runs and how CI matrix jobs pick shards; mistakes in env or flag interaction could run the wrong subset of tests, though behavior without matrix env is largely unchanged.

Overview
depot tests run and depot tests split now resolve --index and --total from DEPOT_MATRIX_JOB_INDEX and DEPOT_MATRIX_JOB_TOTAL when those flags are omitted, so matrix jobs can shard without repeating shard flags. Explicit flags still win; you can mix one flag with one env value. --total 1 alone forces shard 0/1, ignores matrix index, and keeps passthrough behavior (no SplitTests, candidates optional on run).

Split mode is only enabled when total > 1 (runSplitRequested), replacing the older rule that treated any shard-related flag as a split request. Matrix env values are validated (integer parsing, index/total must be set together, same validateShard rules as flags).

^{Reviewed by Cursor Bugbot for commit 88f8591. Bugbot is set up for automated code reviews on this repo. Configure here.}

[linear-code]

DEP-5367

[cursor]

Approved: Cursor Bugbot and Cursor Security Agent both completed successfully with no findings requiring human review. No reviewers assigned.

  

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