[GH] (texera/main): Workflow run "npm_and_yarn in /frontend for fast-uri - Update #1477331451" is working again!

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The GitHub Actions job "npm_and_yarn in /frontend for fast-uri - Update #1477331451" on texera.git/main has succeeded.
Run started by GitHub user dependabot[bot] (triggered by dependabot[bot]).

Head commit for run:
3149eb5ce55de0db029bdba2c3106684e6c65a74 / Xinyuan Lin <xi...@uci.edu>
revert: route all Codecov flags through the deferred workflow_run upload (#6840)

### What changes were proposed in this PR?

Reverts #6730 ("ci: route all Codecov flags through the deferred
workflow_run upload").

Together with the already-merged #6837 (revert of #6824), this fully
backs out the deferred `workflow_run` Codecov upload: the inline
`codecov/codecov-action` steps are restored in `build.yml` for every
flag, and both `codecov-upload.yml` and
`.github/scripts/stage-codecov.sh` are removed.

The deferred approach regressed coverage — because the upload moved out
of the Required Checks run, Codecov built each PR's comment from
carryforward before the deferred upload landed, so every PR showed its
own flag as `Carriedforward` instead of a fresh delta (full analysis in
#6839). Restoring inline uploads brings back fresh per-PR coverage.

### Any related issues, documentation, discussions?

Closes #6839

### How was this PR tested?

Revert of CI-only workflow changes; no product code is affected.
Verified on the branch that `build.yml` restores the inline
`codecov-action` upload steps for all flags (frontend, amber,
amber-integration, the six platform services, pyamber, agent-service)
and that `codecov-upload.yml` and `stage-codecov.sh` are gone. Fresh
(non-carryforward) coverage is confirmed once this merges and the next
PR's Codecov comment again shows its own flag with a delta.

### Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8 [1M context])

Report URL: https://github.com/apache/texera/actions/runs/30049146762

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