chore(bidi): add support for fetching request bodies

[hbenl]

Fixes the following tests in Firefox:

• tests/library/har.spec.ts:
  • "should include postData"
  • "should include binary postData"
  • "should include form params"
• tests/page/network-post-data.spec.ts:
  • "should return correct postData buffer for utf-8 body"
  • "should return post data w/o content-type"
  • "should throw on invalid JSON in post data"
  • "should return post data for PUT requests"
  • "should get post data for file/blob"
  • "should get post data for navigator.sendBeacon api calls"
• tests/page/page-network-request.spec.ts:
  • "should parse the data if content-type is application/x-www-form-urlencoded"
  • "should parse the data if content-type is application/x-www-form-urlencoded; charset=UTF-8"
  • "should parse the json post data"
  • "should return multipart/form-data"
  • "should return postData"
  • "should work with binary post data"
  • "should work with binary post data and interception"
• tests/page/page-request-intercept.spec.ts:
  • "request.postData is not null when fetching FormData with a Blob"
• tests/page/page-route.spec.ts:
  • "should intercept when postData is more than 1MB"

[github-actions]

Test results for "MCP"

2588 passed, 116 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

1 failed
❌ [playwright-test] › runner.spec.ts:118 › should ignore subprocess creation error because of SIGINT @macos-latest-node18-2

4 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1079 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node18`
⚠️ [firefox-page] › page/page-goto.spec.ts:83 › should work with Cross-Origin-Opener-Policy `@firefox-ubuntu-22.04-node18`
⚠️ [firefox-page] › page/page-wait-for-function.spec.ts:104 › should work with strict CSP policy `@firefox-ubuntu-22.04-node18`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:719 › should partition action tree state by test `@macos-latest-node18-2`

40317 passed, 789 skipped

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Merge workflow run.

[yury-s]

We cannot do asynchronous staff in an event handler, the dispatcher is not waiting for the listener to complete and will fire subsequent events. With the current implementation the request may not be in the map when _onResponseStarted arrives and we'll miss the response.

Ideally we'd have an asynchronous request.body() and not have to wait here at all, but all the postData* methods in playwright API today are synchronous (which is a regret).

Alternatively, we could buffer all network events for a given request. However, to preserve their relative order we’d effectively need to buffer all network events, which complicates things. It also means unrelated events could interleave differently. E.g. a console message might appear before a network request. If we don't buffer up all the requests and wait for the request bodies independently, users may see sub-resource requests before the main request.

Considering all the above, we should add an async getter for the request body and have Bidi implement only that. Let's postpone this change until that is done. Filed #38150 for this.