fix: preserve multiline fields during import

[1fanwang]

influx_inspect export preserves literal newlines inside quoted string fields. influx -import counted those physical lines toward its 5,000-line batch, so a batch could end inside a field and fail to round trip with unbalanced quotes.

The importer now batches complete line-protocol records. Record framing uses models.ScanLine for quotes and escapes, rejects a single record above the server's default 25 MB request-body limit, and flushes valid records collected before a later read error.

Fixes #27589

Testing

5,000-line red/green repro and package output

Setup:
$ go build -o /tmp/influx-pkg-config github.com/influxdata/pkg-config

Before (afddccc2baec3ffc771cd1a120dbe8ace28f7bc2):
$ PKG_CONFIG=/tmp/influx-pkg-config go test ./importer/v8 -run '^TestImporter_MultilineFieldsAcrossBatchBoundary$' -count=1 -v
hhhhhhhhhh': unbalanced quotes
import failed: 6012 points were not inserted
--- FAIL: TestImporter_MultilineFieldsAcrossBatchBoundary (0.01s)
FAIL github.com/influxdata/influxdb/importer/v8 0.661s

After (2615fa1c145ed0affb433727b07ec82949e21d5a):
$ PKG_CONFIG=/tmp/influx-pkg-config go test ./importer/v8 -run '^TestImporter_(MultilineFieldsAcrossBatchBoundary|FlushesBatchOnReadError|UnterminatedFinalPoint)$' -count=1 -v
--- PASS: TestImporter_MultilineFieldsAcrossBatchBoundary (0.01s)
--- PASS: TestImporter_UnterminatedFinalPoint (0.00s)
--- PASS: TestImporter_FlushesBatchOnReadError (0.00s)
PASS
ok github.com/influxdata/influxdb/importer/v8 0.763s

$ PKG_CONFIG=/tmp/influx-pkg-config go test ./importer/v8 ./models -count=1
ok github.com/influxdata/influxdb/importer/v8 0.532s
ok github.com/influxdata/influxdb/models 2.293s

$ PKG_CONFIG=/tmp/influx-pkg-config go test -race ./importer/v8 -count=1
ok github.com/influxdata/influxdb/importer/v8 1.732s

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[gwossum]

Thanks for the PR! This correctly identifies the issue, that influx import assumes 1 line == 1 point and disregards quoting.

My main request is to re-use models.scanLine instead of copying it. Most of the rest is matching the projects modern test code style, as well as a few questions.

[gwossum]

Almost there! Just a couple of tweaks and this can get merged. I also added some non-blocking comments. Nothing needs to be done on those for approval.

[gwossum]

I like that a test was added for this.

[gwossum]

LGTM. Thanks for the PR, and congratulations on your first InfluxDB PR!