fix(core): surface refusal category and explanation on content filter

[cyllas]

Issue for this PR

Closes #35736

Type of change

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

What does this PR do?

When Anthropic returns stop_reason: "refusal", opencode maps it to a
content-filter finish and shows one hardcoded message — "The response was
blocked by the provider's content filter" — dropping the stop_details the
provider sends. As #35736 notes, that makes every refusal look identical even
though the categories call for different responses.

Anthropic's stop_details.category can be cyber, bio, frontier_llm, or
reasoning_extraction (see Anthropic's "Refusals and fallback" docs).
@ai-sdk/anthropic already surfaces these on the finish event's
providerMetadata.anthropic.stopDetails, but they were never read.

This PR:

• extends ContentFilterError with optional category and explanation
• reads the refusal details from the finish event in the session processor
• includes them in the error message (e.g. Response refused by provider (reasoning_extraction): ...) when present, falling back to the current
  message when they are not

No behavior change when stop_details is absent — every non-refusal stop, or a
refusal with no named category.

How did you verify your code works?

• Reproduced a real reasoning_extraction refusal from claude-fable-5 end to
  end. The surfaced error changed from the generic message to
  Response refused by provider (reasoning_extraction): <explanation>. Confirmed
  providerMetadata.anthropic.stopDetails arrives on the step-finish event with
  category/explanation and is read correctly.
• Added test/session/content-filter.test.ts covering the metadata parsing
  (category + explanation, partial, and the empty/absent cases); bun test passes.
• bun typecheck passes in packages/core and packages/opencode.

Screenshots / recordings

N/A (error text change).

Checklist

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