feat: add request.body async getters

[hbenl]

Fixes #38150 and the following tests:

with Chrome/CDP:

• page/network-request-body.spec.ts:
  • "should get request body for file/blob"
  • "should get request body for navigator.sendBeacon api calls"

with BiDi:

• page/network-request-body.spec.ts:
  • "should return correct request body buffer for utf-8 body"
  • "should return request body w/o content-type"
  • "should throw on invalid JSON in post data"
  • "should return body for PUT requests"
  • "should get request body for file/blob"
  • "should get request body for navigator.sendBeacon api calls"

[hbenl]

The failing BiDi tests are all passing locally. CI test results seem flakier than they used to be, perhaps due to the increased number of workers.

[github-actions]

Test results for "tests 1"

1 failed
❌ [default-trace] › debug-tests.spec.ts:516 › should not pause at the end of a setup test @vscode-extension

3 flaky

⚠️ [chromium-page] › page/page-request-continue.spec.ts:754 › propagate headers cross origin redirect after interception `@chromium-ubuntu-22.04-node24`
⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1082 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node20`
⚠️ [firefox-page] › page/page-wait-for-function.spec.ts:104 › should work with strict CSP policy `@firefox-ubuntu-22.04-node20`

34489 passed, 696 skipped

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Merge workflow run.

[github-actions]

Test results for "MCP"

5 failed
❌ [chromium] › mcp/launch.spec.ts:80 › persistent context @mcp-macos-15
❌ [firefox] › mcp/test-debug.spec.ts:167 › test_debug (pause/snapshot/resume) @mcp-macos-15
❌ [firefox] › mcp/test-setup.spec.ts:105 › test_debug should run global setup and teardown @mcp-macos-15
❌ [webkit] › mcp/http.spec.ts:263 › http transport shared context @mcp-macos-15
❌ [webkit] › mcp/planner.spec.ts:69 › planner_setup_page seed resolution @mcp-macos-15

2817 passed, 116 skipped

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Merge workflow run.