fix: accumulate tokens across multi-step tool calls instead of overwriting

[KonstantinMirin]

Issue for this PR

Fixes #21913

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

In multi-step tool-call messages, processor.ts overwrites assistantMessage.tokens on each finish-step event instead of accumulating additive fields. Only the last step's output, reasoning, and cache.write survive. The fix: accumulate output, reasoning, cache.write with += across steps; replace input and cache.read with the latest step's values (they're snapshots).

Additionally fixes custom provider limit.context defaulting to 0 (now 128_000) and limit.output defaulting to 0 (now 4_096), which broke context % display and disabled auto-compaction.

Key changes:

• processor.ts: field-wise token accumulation instead of wholesale overwrite
• index.ts: getUsage() derives total from components (input + output + reasoning + cache.read + cache.write) instead of using API's totalTokens (which double-counts cached tokens)
• message-v2.ts: add promptSize() and totalSize() named helpers
• overflow.ts: use MessageV2.totalSize() for compaction threshold (output/reasoning are part of next turn's context)
• provider.ts: named constants DEFAULT_CONTEXT_LIMIT / DEFAULT_OUTPUT_LIMIT instead of magic 0
• agent.ts: include cache.write in ACP usage reporting

How did you verify your code works?

1903 existing tests pass + 12 new tests covering:

• promptSize/totalSize helpers (3 unit tests)
• isOverflow pure unit tests (5 tests: disabled, zero context, underground, overflow, custom reserved)
• getUsage total derivation from components (1 test)
• 3-step accumulation simulation mirroring processor.ts logic (1 test)
• Provider limit defaults (1 test, updated from 0 to 128_000/4_096)
• Overflow semantics test (1 test, updated from total-based to totalSize-based)

Screenshots / recordings

N/A — no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR