Honor max_output_length for custom plugin stdout capture

[kv-cache]

What this PR does / why we need it:

Custom-plugin stdout is read into a hardcoded 4 KiB buffer
(maxCustomPluginBufferBytes = 1024 * 4) and only then truncated to the
configured max_output_length, so the effective limit is
min(4096, max_output_length). Any plugin configured with
max_output_length > 4096 is silently capped at 4096 bytes — the configured
value has no effect. For a plugin that emits JSON, the cut lands mid-token and
produces invalid output that a strict consumer discards entirely.

This drives the stdout read limit from max_output_length, bounded by a 1 MiB
safety ceiling to protect against a runaway plugin. stderr keeps a small fixed
cap since it is used only for diagnostic logging (logPluginStderr) and is never
included in the returned output. No behavior change for the default config
(max_output_length = 80).

Which issue(s) this PR fixes:

NONE

Special notes for your reviewer:

Adds TestPluginRunCaptureRespectsMaxOutputLength plus a fixture that emits
16384 bytes. With max_output_length = 8192 the captured output must be exactly
8192 bytes — this fails on the current code (capped at 4096) and passes with the
fix. go test -short -race ./pkg/custompluginmonitor/... passes; gofmt/vet clean.

This raises the capture ceiling so a plugin's configured max_output_length
takes effect; it does not change that output beyond max_output_length is still
byte-truncated.

/kind bug

Does this PR introduce a user-facing change?:

Custom plugin monitor now honors a `max_output_length` larger than 4096 bytes. Previously plugin stdout was silently capped at a hardcoded 4 KiB internal buffer regardless of the configured `max_output_length`.

Note:

Made with AI

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: kv-cache / name: Vineeth Rao Kanaparthi (25f099b)

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