[docs] decision: network responses expose their original body and request timing

[AutomatedTester]

💥 What does this PR do?

Proposes a design decision record: network responses expose their original body and request timing.

The high-level network handlers can mutate/stub traffic but can't read the original response body or expose request timing — the two most common monitoring needs. Both are now supported by BiDi: network.addDataCollector + network.getData capture response (and increasingly request) bodies, and FetchTimingInfo (13 fields) already rides on every request. This record proposes body()/text()/json() and fetch-then-patch on the Response wrapper (via managed data collectors) plus timing on the Request wrapper. Because data collectors are newer and unevenly implemented across browsers, body-read raises a typed "unsupported" error with a capability gate rather than failing silently; timing is always available.

🔧 Implementation Notes

• Follows the docs/decisions/ process from [docs] add design decision record process and template #17665 — depends on [docs] add design decision record process and template #17665 landing (adds only docs/decisions/0004-*.md).
• Body-read is gated on browser support (Firefox supports response, added request; Chromium supported, consumed by Puppeteer; known FF redirect-timing edge cases). The binding manages collector lifecycle so users don't juggle collector ids.
• Timing needs no collector and can land ahead of body-read. No wire-protocol change.

🤖 AI assistance

• AI assisted (complete below)
  • Tool(s): Claude Code
  • What was generated: Initial draft of the decision record, derived from a CDDL-validated comparison of the Python BiDi surface against Playwright, plus a check of browser support for data collectors; revised through discussion and review
  • I reviewed all AI output and can explain the change

💡 Additional Considerations

One of a set of BiDi ergonomics decision records proposed together. Cross-binding convergence is tracked in the binding-status table.

🔄 Types of changes

• Documentation (design decision record)