feat: make webfetch max response size configurable

[RaviTharuma]

Issue for this PR

Closes #15459

Type of change

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

What does this PR do?

Webfetch currently hard-caps the HTTP body at 5MB. This PR adds webfetch.max_response_size in ConfigV1 (positive int, bytes). The default stays 5MB. Larger pages can be fetched without editing source.

How did you verify your code works?

Unit tests for resolveMaxResponseSize in packages/opencode.

Screenshots / recordings

N/A — not a UI change.

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found one potentially related PR:

PR #45937: fix(webfetch): preserve specific errors and preview oversized responses (fixes #45929)

• Why it's related: This PR also addresses webfetch response handling, specifically dealing with oversized responses. It may be addressing similar issues with response size limits that your PR configures.

You may want to check if PR #45937 is already merged or if there's any overlap in the functionality being addressed.

[Copilot]

🟡 Changes recommended

The new configurable size limit still allows fully buffering oversized responses (when Content-Length is missing/incorrect) and needs input validation/error-message improvements to make the feature reliable and diagnosable.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds configuration support for WebFetch’s maximum HTTP response size so users can raise the previous 5MB hard-cap via ConfigV1 and an environment variable, while keeping the default at 5MB.

Changes:

• Introduces webfetch.max_response_size in ConfigV1 (bytes, positive int) with a default of 5MB.
• Updates the webfetch tool to resolve the limit from config/env and enforce it during fetch.
• Adds unit tests for resolveMaxResponseSize precedence and env parsing behavior.

File summaries

File	Description
packages/opencode/src/tool/webfetch.ts	Adds max-response-size resolution (config/env/default) and applies it to WebFetch size checks.
packages/opencode/test/tool/webfetch.test.ts	Adds unit tests for limit resolution behavior and env var handling.
packages/core/src/v1/config/config.ts	Extends ConfigV1 schema with webfetch.max_response_size (PositiveInt, bytes) and documentation.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 3
• Review effort level: Lite

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[RaviTharuma]

@copilot-pull-request-reviewer re-review: Content-Length is enforced before buffering; PositiveInt config rejects 0/negative; oversized error text is the existing webfetch message with the configured cap.