fix(harness): formatValue's truncation marker could push output past VALUE_CAP

[teyrebaz33]

Primary change type

• Bug fix
• Documentation
• Feature
• Tests
• Dependency update
• Maintenance or refactor

Problem and motivation

formatValue (packages/harness/web/src/lib/extract-step-context.ts) is documented as producing output "capped at VALUE_CAP characters," but it sliced content to exactly VALUE_CAP and then appended a truncation marker AFTER the slice — so the total could exceed VALUE_CAP by the marker's length. This is the fourth instance of the same bug class found in a repo-wide sweep; the other three (truncateForPayload/clip/clamp in packages/harness/src/core/) are fixed in #656 — this one lives in a different subsystem (the web frontend's run-inspector Debug/Explain macro context builder), so it's a separate PR.

Summary and scope

Unlike the three fixed in #656, this marker (\n… (truncated, N chars total)) reports the value's original total length, which doesn't change with where the slice lands — so the fix here is a single subtraction, not a converging loop: the marker's length is fixed once text.length is known, so there's nothing to iterate on. Reserve the marker's length within VALUE_CAP before slicing.

Out of scope: the three packages/harness/src/core/ instances (separate PR, #656); no dependency changes.

Related work

Related issue or discussion: #674

Validation

# packages/harness: vitest run web/src/lib/extract-step-context.test.ts
51/51 passing
tsc --noEmit (server tsconfig): clean
tsc --noEmit -p web/tsconfig.json: clean
# web/ is excluded from this package's eslint config (.eslintrc.cjs ignorePatterns),
# pre-existing and unrelated to this change

Tests and documentation

Updated the one existing test whose exact-match assertion had encoded the old (over-budget) output as the expected value, with the new expected value computed independently. No user-facing documentation changes needed.

Compatibility and release impact

• Breaking or externally visible changes: None. formatValue is not exported; extractStepContext's signature and return type are unchanged. Truncated output near the cap gets slightly shorter (by the marker's length) to actually respect the documented bound.
• Changeset: Added (.changeset/fix-formatvalue-budget-overrun.md), patch bump for @sapiom/harness.

Security

• I have not included secrets, credentials, private data, or unsanitized logs.
• This pull request does not publicly disclose a suspected vulnerability. I
  will follow the
  Security Policy for
  private reporting.

AI assistance

• I did not use AI assistance for this change.
• I used AI assistance and have described it below.

I used Claude (Anthropic) as a coding assistant throughout: it helped find the bug (via a repo-wide grep sweep for the same marker pattern), draft the fix and the updated test, and run the verification commands quoted above. I reviewed and ran every command myself, read and understood the resulting diff line by line, and can explain and maintain every change in this PR.

Checklist

• I read CONTRIBUTING.md, and this contribution follows the direct-PR or issue-first policy.
• This pull request addresses one focused problem and contains no unrelated cleanup.
• I added or updated tests, or explained above why tests are not applicable.
• I ran the relevant build, typecheck, lint, and test commands, or explained
  any N/A checks above.
• I updated documentation for user-facing changes, or marked it N/A above.
• I added a Changeset for a published-package change, or explained why it is not applicable.
• I can explain and maintain every submitted change, including any AI-assisted work.