[Provider diagnostics] Preserve both process streams on batch failure

Problem

BatchGraphSession drops a batch producer's stdout whenever stderr contains
any text. Build wrappers legitimately divide their output across both streams,
so a benign notice on stderr can hide the only actionable failure on stdout.

The Gson index-time cell in run 33377586947 is the concrete reproduction.
javac-graph exited with code 1. The retained message consists only of three
Picked up JAVA_TOOL_OPTIONS notices from stderr, although scip-java runs a
real Maven build whose failure summary is written to stdout. The current
failureDetail returns stderr immediately when it is non-empty and consults
the bounded stdout tail only when stderr is empty.

Consequence surface

The shared session owns every one-shot SCIP, sidecar, Go, Lua, and Java process.
The defect affects CLI errors, strict-provider fallback warnings, resident
retry diagnosis, experiment artifacts, and index-time logs. It is not specific
to Gson or Maven.

Stdout can also be an artifact channel and both streams may be large. A fix
must retain bounded diagnostic tails without embedding an artifact or allowing
failure output to grow without limit.

Approach

Preserve separately attributed, bounded tails for both non-empty streams. A
stderr-only or stdout-only failure remains concise. When both are non-empty,
include both labels and tails under one fixed total diagnostic budget, favoring
the end of each stream because build tools print their verdict last.

Acceptance and verification

• A producer with a benign stderr notice and a stdout failure retains both and
  identifies which stream supplied each text.
• Existing stderr-only, stdout-only, silent, short-output, long-output,
  cancellation, output-limit, and process-tree cleanup behavior remains
  covered.
• Truncation is explicit only when bytes were omitted, and the combined message
  has a fixed upper bound.
• Successful artifact stdout is never copied into a diagnostic.
• The next Gson run exposes the actual Maven cause if the route still fails.
• The focused BatchGraphSession/SCIP tests, pnpm build, pnpm test, and
  pnpm coverage pass on the same head.

Coordination

This issue improves the evidence needed to finish #74 but does not close Java's
semantic, fixture, JDT, or performance acceptance items by itself.

Locate BatchGraphSession and its failureDetail implementation, then read the focused BatchGraphSession/SCIP tests covering stream handling and output limits. Verify that both bounded, labeled tails are retained under one limit while artifact stdout and existing cleanup behavior remain unaffected; run the focused tests, pnpm build, pnpm test, and pnpm coverage.