Add network request mocking keyword family: `Mock Response`, `Abort Requests`, `Modify Requests`, `Remove Route`, `Remove All Routes`

[Snooz82]

Use case

Network mocking is the single biggest Playwright capability that Browser library does not expose at all today: there is no keyword, no wrapper code, and no proto RPC for page.route/context.route. Users cannot:

• Stub an API endpoint to test the frontend in isolation from the backend (return fixture JSON for /api/**).
• Test error paths deterministically (make /api/orders return a 500 without breaking the real backend).
• Block unwanted requests (abort analytics beacons, ads, or all images to speed up suites).
• Modify outgoing requests (inject/override headers, redirect a request to a different URL, change method or post data).

There is currently no workaround inside the library — users who need mocking must run an external proxy or a separate mock server, which defeats the point of an integrated browser automation library. Nearly every real-world suite eventually needs at least one of these capabilities, which makes this the flagship feature of the network milestone.

Playwright's route API is callback-based, which does not fit the keyword model. The design decision (see docs/plan/playwright-feature-gap-plan.md) is to reshape it into declarative rules stored node-side, exposed as separate verb keywords (not a single Route keyword with an action enum). The library already uses this reshaping pattern in Add Locator Handler Custom.

Proposed keyword / arguments

All keywords take a matcher (URL glob pattern or regex) with the same semantics as Wait For Request / Wait For Response (reusing deserializeUrlOrPredicate). All rule-creating keywords default to scope=Context so mocks survive navigations and apply to popups/new pages; pass named-only scope=Page to narrow to the current page.

Mock Response       matcher    *, status=200    body=None    json=None    path=None    content_type=None    headers=None    times=None    scope=Context
Abort Requests      matcher    *, error_code=failed    times=None    scope=Context
Modify Requests     matcher    *, headers=None    url=None    method=None    post_data=None    scope=Context
Remove Route        matcher
Remove All Routes

• Mock Response fulfills matching requests. Exactly one of body, json, path (file with response body) provides the payload; json sets content-type: application/json automatically.
• Abort Requests aborts matching requests with the given Playwright error code (failed, aborted, accessdenied, connectionrefused, internetdisconnected, ...).
• Modify Requests continues matching requests with overrides (extra/overridden headers, rewritten URL, method, or post data).
• times= limits how many times a rule fires before it is removed automatically (maps to the route option times).
• Optional stretch goal: named-only base_on_server_response=True on Mock Response — fetch the real response first (route.fetch) and fulfill with the given fields merged on top, so users can mutate real responses (e.g. patch one JSON field).

*** Test Cases ***
Frontend Renders Empty User List
    Mock Response    **/api/users    json={"users": []}
    Go To    ${APP_URL}
    Get Text    id=user-list-empty    ==    No users found

Error Banner On Server Failure
    Mock Response    **/api/orders    status=500    json={"error": "boom"}    times=1
    Click    id=load-orders
    Get Text    css=.error-banner    contains    Something went wrong

Block Analytics For Whole Suite
    Abort Requests    /.*google-analytics.*/

Inject Auth Header Only On Current Page
    Modify Requests    **/api/**    headers={"Authorization": "Bearer ${token}"}    scope=Page

Cleanup
    Remove Route    **/api/users
    Remove All Routes

Playwright API

• browserContext.route / page.route (incl. the times option)
• route.fulfill — Mock Response
• route.abort — Abort Requests
• route.continue — Modify Requests
• route.fetch — optional base_on_server_response stretch
• browserContext.unroute / browserContext.unrouteAll — Remove Route / Remove All Routes

Implementation notes

• New RPCs in protobuf/playwright.proto for registering/removing route rules (rule spec message: matcher, action, action payload, times, scope).
• Node-side rule store in node/playwright-wrapper: one real route() handler per registered matcher that executes the stored declarative action; matcher parsing reuses the existing deserializeUrlOrPredicate from the Wait For Request/Response path. Rule ordering semantics (last registered wins, Playwright-style) must be defined and documented.
• New Python keywords in Browser/keywords/network.py, docs, inv build stub regen.
• New atest suite against the dynamic test app (fixture endpoints for fulfill/abort/continue, times=, scope behavior across New Page/navigation).
• This is a full milestone on its own (see plan: Wave 2, "Network mocking").

Backwards compatibility

Purely additive: five new keywords, no changes to any existing keyword or return value. Existing suites are unaffected unless they call the new keywords.