fix(web): persist docs English locale selection

[SpencerJung]

Summary

• persist oc_locale=en when docs navigation selects the default English locale via an unprefixed docs URL
• keep locale alias redirects in one helper and add focused tests for English selection, alias handling, and cookie parsing

Closes #29326

Verification

• lsp_diagnostics: packages/web/src/middleware.ts
• lsp_diagnostics: packages/web/src/middleware-locale.ts
• lsp_diagnostics: packages/web/src/middleware-locale.test.ts
• bun test --cwd packages/web src/middleware-locale.test.ts
• bun run --cwd packages/web build

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential duplicate found:

• PR fix(web): persist docs English locale selection #29351: fix(web): persist docs English locale selection - fix(web): persist docs English locale selection #29351

This PR has the same exact title as the current PR (#29381) and appears to address the same issue (persisting English locale selection in the docs). This is likely a duplicate that needs to be addressed—either this PR should be closed or PR #29351 should be reviewed if it was already merged/closed.