Add --tag option to set tag for a run

[nprizal]

Summary

• Adds --tag flag and BUILDKITE_TEST_ENGINE_TAGS env var to bktec run to tag the run when uploading to Test Engine
• Tags are key=value pairs sent as tags[key]=value multipart fields in the upload body
• Brings feature parity with test-collector-buildkite-plugin for upload-level tagging

Usage

# CLI flags (repeatable)
bktec run --tag env=production --tag region=us-east-1

# Environment variable (comma-separated)
export BUILDKITE_TEST_ENGINE_TAGS="env=production,region=us-east-1"

Notes for reviewer

• The comma-split in the flag Action is needed because DisableSliceFlagSeparator: true on the run command prevents the CLI library from splitting env var values automatically. There's no per-flag override for this in urfave/cli/v3.
• Parsing and validation happen in the flag's Action callback in cli.go, not in main.go.
• The README includes an "Upload test results to Test Engine" section. This overlaps with changes in te-6006-update-bktec-docs — a merge conflict is expected there, just keep whichever version includes the --upload-tags docs.

Closes TE-6027

[pda]

bktec run --upload-tags env=production --upload-tags region=us-east-1

The plural on repeated --upload-tags bugs me a bit. If each option takes one tag, then --upload-tag perhaps is better?

bktec run --upload-tag env=production --upload-tag region=us-east-1

And further, I'm not sure we need to specify that it's an upload tag here. They're just tags that end up on all the executions. So maybe just --tag?

bktec run --tag env=production --tag region=us-east-1

That'd be my preference ☝🏼

[nprizal]

And further, I'm not sure we need to specify that it's an upload tag here. They're just tags that end up on all the executions. So maybe just --tag?

bktec run --tag env=production --tag region=us-east-1

Will people confuse this as "run tests that has these tags"? We have tag-filters for pytest that does that, so I use upload-tags to avoid confusion. Also, in Rspec --tag is used to filter tests with specific tags.

[buildsworth-bk-app]

Small, focused addition and the upload path looks correct — tags[key]=value mirrors the existing run_env[...] multipart fields, and the env-var binding test plus the multipart-body test cover the main paths.

One non-blocking note inline about comma handling in tag values.

The one open item is really the flag name (--tag vs something more specific) — you and @pda are already on that thread, so I'll leave that call to you two rather than pile on.

Note: this PR has no risk: label.

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