chore(ci): restore Codecov carryforward after flag rebaseline

[mengw15]

What changes were proposed in this PR?

Step 2 (final) of the Codecov carryforward reset started in #6852.

#6852 removed flag_management.default_rules.carryforward and let main rebuild
once under the current flag set, flushing the stuck carryforward sessions left by
the reverted deferred-upload experiment (#6730/#6824). This PR re-enables
carryforward so single-stack PRs keep backfilling non-run flags from the base
report.

Net effect versus pre-#6852: identical functional config — the whole exercise
leaves no lasting config change; it only rebaselines Codecov's carryforward data
on main.

⚠️ Verification gate. This PR's own Codecov comment is the A/B test for the reset:

• chore(ci): disable Codecov carryforward for a one-time flag rebaseline #6852 (carryforward off) showed non-run flags as ?.
• This PR (carryforward on) must show each PR's own flag as a fresh delta
  again, and must NOT resurrect any stale flag as a 0.00% carried-forward row.

Do not merge until the Codecov comment here is confirmed clean.

Any related issues, documentation, discussions?

Follows #6852. Closes #6851.

How was this PR tested?

Config-only change. Validated codecov.yml parses as valid YAML and that the
functional blocks (flag_management, coverage, comment) match the pre-#6852
configuration, restoring carryforward: true. Behavioral verification is the
Codecov comment on this PR (see the verification gate above).

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

Generated-by: Claude Code (claude-opus-4-8)

[github-actions]

Automated Reviewer Suggestions

Based on the git blame history of the changed files, we recommend the following reviewers:

• No candidates found from git blame history.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.30%. Comparing base (a351f44) to head (9167329).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6856   +/-   ##
=========================================
  Coverage     77.30%   77.30%           
  Complexity     3524     3524           
=========================================
  Files          1161     1161           
  Lines         45922    45922           
  Branches       5101     5101           
=========================================
+ Hits          35501    35502    +1     
+ Misses         8840     8839    -1     
  Partials       1581     1581           

Flag	Coverage Δ
access-control-service	70.00% <ø> (ø)
agent-service	76.76% <ø> (ø)
amber	69.10% <ø> (+<0.01%)	⬆️
computing-unit-managing-service	20.49% <ø> (ø)
config-service	66.66% <ø> (ø)
file-service	67.21% <ø> (ø)
frontend	82.58% <ø> (ø)
notebook-migration-service	78.94% <ø> (ø)
pyamber	92.15% <ø> (ø)
workflow-compiling-service	55.14% <ø> (ø)

☔ View full report in Codecov by Harness.
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[github-actions]

⚠️ Benchmark changes need a look

🟢 5 better · 🔴 2 worse · ⚪ 8 noise (<±5%) · 0 without baseline

Compared against main a351f44 benchmarked on this same runner, so the delta is largely free of cross-runner hardware noise. The "7d avg" column still reflects the gh-pages dashboard. Treat <±5% as noise unless repeated.

Dashboard · Run

	config	throughput	MB/s	latency	max Δ latest / 7d
🔴	bs=10 sw=10 sl=64	438	0.268	21,285/32,184/32,184 us	🟢 -5.5% / 🔴 +101.0%
🟢	bs=100 sw=10 sl=64	962	0.587	102,191/117,999/117,999 us	🟢 -27.9% / 🔴 +10.9%
⚪	bs=1000 sw=10 sl=64	1,083	0.661	922,618/966,326/966,326 us	⚪ within ±5% / 🟢 -8.0%

Baseline details

Latest main a351f44 from same runner

config	metric	PR	latest main	7d avg	Δ latest	Δ 7d
bs=10 sw=10 sl=64	throughput	438 tuples/sec	450 tuples/sec	767.66 tuples/sec	-2.7%	-42.9%
bs=10 sw=10 sl=64	MB/s	0.268 MB/s	0.274 MB/s	0.469 MB/s	-2.2%	-42.8%
bs=10 sw=10 sl=64	p50	21,285 us	22,522 us	12,623 us	-5.5%	+68.6%
bs=10 sw=10 sl=64	p95	32,184 us	30,571 us	16,011 us	+5.3%	+101.0%
bs=10 sw=10 sl=64	p99	32,184 us	30,571 us	18,856 us	+5.3%	+70.7%
bs=100 sw=10 sl=64	throughput	962 tuples/sec	890 tuples/sec	998.44 tuples/sec	+8.1%	-3.6%
bs=100 sw=10 sl=64	MB/s	0.587 MB/s	0.543 MB/s	0.609 MB/s	+8.1%	-3.7%
bs=100 sw=10 sl=64	p50	102,191 us	102,831 us	100,463 us	-0.6%	+1.7%
bs=100 sw=10 sl=64	p95	117,999 us	163,770 us	106,400 us	-27.9%	+10.9%
bs=100 sw=10 sl=64	p99	117,999 us	163,770 us	117,712 us	-27.9%	+0.2%
bs=1000 sw=10 sl=64	throughput	1,083 tuples/sec	1,128 tuples/sec	1,034 tuples/sec	-4.0%	+4.7%
bs=1000 sw=10 sl=64	MB/s	0.661 MB/s	0.689 MB/s	0.631 MB/s	-4.1%	+4.7%
bs=1000 sw=10 sl=64	p50	922,618 us	883,952 us	973,294 us	+4.4%	-5.2%
bs=1000 sw=10 sl=64	p95	966,326 us	942,731 us	1,019,213 us	+2.5%	-5.2%
bs=1000 sw=10 sl=64	p99	966,326 us	942,731 us	1,049,896 us	+2.5%	-8.0%

Raw CSV

config_idx,batch_size,schema_width,string_len,num_batches,total_ms,total_tuples,total_bytes,tuples_per_sec,mb_per_sec,lat_p50_us,lat_p95_us,lat_p99_us
0,10,10,64,20,456.23,200,128000,438,0.268,21285.13,32183.87,32183.87
1,100,10,64,20,2078.95,2000,1280000,962,0.587,102191.28,117999.02,117999.02
2,1000,10,64,20,18464.29,20000,12800000,1083,0.661,922618.46,966325.50,966325.50