chore(locators): rewrite locator parser with per-language parsers

[Yury Semikhatsky (yury-s)]

Summary

• Replace the regex-based locator parser with a tokenizer and a recursive descent parser per language (JS, Python, Java, C#) that produces a ParsedSelector directly; each language maps its method names to shared call handlers.
• The parser validates methods, arguments and options itself, so the round-trip check through asLocators is removed. Also parses and(), or() and locator(locator).
• Support multiple regex flags (i, m, s) in Python, Java and C# locators; the generators no longer drop m and s.
• Redundant engine prefixes are normalized, e.g. locator('css=.foo') now yields .foo.

References #42891

[github-actions]

Test results for "tests 1"

7 flaky

⚠️ [chromium-library] › library/browsercontext-page-event.spec.ts:160 › should work with Shift-clicking `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/browsercontext-page-event.spec.ts:173 › should work with Ctrl-clicking `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/popup.spec.ts:260 › should not throw when click closes popup `@chromium-ubuntu-22.04-node24`
⚠️ [chromium-library] › library/chromium/chromium.spec.ts:179 › serviceWorker(), and fromServiceWorker() work `@chromium-ubuntu-22.04-node20`
⚠️ [chromium-library] › library/video.spec.ts:762 › screencast › should work with video+trace `@chromium-ubuntu-22.04-node20`
⚠️ [firefox-library] › library/browsercontext-cookies-third-party.spec.ts:257 › third party 'Partitioned;' cookies `@firefox-ubuntu-22.04-node20`
⚠️ [firefox-library] › library/browsercontext-cookies-third-party.spec.ts:470 › top level 'Partitioned;' cookie and same origin iframe `@firefox-ubuntu-22.04-node20`

52181 passed, 1241 skipped

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Merge workflow run.

[github-actions]

Test results for "MCP"

2 failed
❌ [firefox] › mcp/http.spec.ts:105 › http transport browser lifecycle (isolated) @mcp-ubuntu-latest-firefox
❌ [firefox] › mcp/cli-core.spec.ts:57 › click link @mcp-windows-latest-firefox

8703 passed, 1474 skipped

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Merge workflow run.

[github-actions]

Hi, I'm the Playwright bot and I took a look at the failing CI runs here.

🟢 Both failures are pre-existing Firefox MCP flakes — the PR is clear

mcp/http.spec.ts:105 and mcp/cli-core.spec.ts:57 both fail on Firefox regularly on main and on unrelated PRs, with the exact same errors.

Details

This PR only touches packages/isomorphic/locatorParser.ts / locatorGenerators.ts and tests/library/locator-generator.spec.ts. The locator parser is consumed by the recorder and trace viewer to turn locator source text into a selector; neither failing MCP test parses locator strings, and nothing in the diff is Firefox-specific. The "tests 1" report is green (52181 passed, 7 flaky, 0 failed).

Pre-existing flake / infra

• [firefox] › mcp/http.spec.ts:105 › http transport browser lifecycle (isolated) @mcp-ubuntu-latest-firefox — Test timeout of 30000ms exceeded. In the aggregated CI results this test has failed 22 of 2131 runs (1.0%) on firefox, and it also fails on chromium (13/2171), webkit (11/2195) and msedge (4/716). Same timeout error on pushes to main unrelated to this PR, e.g. sha 56c0b49f (run 35755589546), 3bbcbca7 (run 35748035194), bb78937f (run 35138614208), plus other PRs (feat(mcp): add media emulation #42668, fix(screencast): keep overlays above top layer elements #42642, fix(mcp): follow symlinks when checking file access against workspace roots #42628, fix(recorder): keep recordings separate per client on a shared context #42615).
• [firefox] › mcp/cli-core.spec.ts:57 › click link @mcp-windows-latest-firefox — expect(received).toContain(expected) on the ### Page - Page URL: … snapshot, i.e. the snapshot is taken before the click-triggered navigation settles. Failed 14 of 2131 firefox runs (0.7%), and never once on chromium, webkit, chrome or msedge — it's Firefox-on-Windows specific. Identical failure on main sha 0ee695b5 (run 35380366731) and 0d864018 (run 32321251720), and on fix(aria): annotate aria-hidden elements in AI snapshots #42268.

Caveat, as always: this is a statistical argument, not a proof. A rerun of the MCP job should come back green.

_{Triaged by the Playwright bot - agent run}