feat: nightly hardening - browserless response size cap

[mouse-value-add]

Problem

Browserless /content responses were read with an unbounded io.ReadAll, which could allow unexpectedly large rendered pages to consume excessive memory and destabilize MCP requests.

Approach

• Added a hard 2 MiB cap for Browserless rendered HTML payloads (maxHTMLBytes).
• Switched body reads to io.LimitReader(..., maxHTMLBytes+1) to detect overflow deterministically.
• Return a permanent provider error when the cap is exceeded, so callers fail fast instead of retrying large payloads.
• Added a provider test covering oversized payload behavior.

Verification

• go test ./...
• New test: TestRender_ResponseTooLargeIsPermanent in internal/provider/browserless/browserless_test.go

Risks

• Some legitimately large pages may now be rejected when rendered HTML exceeds 2 MiB.
• This is a tradeoff in favor of bounded memory and predictable behavior.

Rollback plan

• Revert this PR to restore previous behavior, or increase maxHTMLBytes if real-world payloads require a higher ceiling.