fix(auth): allow longer reCAPTCHA language tags

[KirtiRamchandani]

Problem

auth.useDeviceLanguage() can set auth.languageCode to real browser language tags longer than six characters, such as en-GB-oxendict. The reCAPTCHA V2 loader rejects those values as auth/argument-error before loading the script.

Root cause

isHostLanguageValid() enforced hl.length <= 6, which only fits short tags like en or en-GB and rejects valid longer browser language tags.

Solution

Remove the arbitrary six-character limit while preserving the existing alphanumeric/hyphen/whitespace whitelist, and add a regression test that verifies en-GB-oxendict is accepted and passed through as the hl parameter.

Tests run

• corepack yarn install --frozen-lockfile --ignore-engines (Node 24 requires ignoring engines; optional re2 native build failed but install completed)
• corepack yarn --cwd packages/auth build:deps
• ..\..\node_modules\.bin\eslint.cmd -c .eslintrc.js src/platform_browser/recaptcha/recaptcha_loader.ts src/platform_browser/recaptcha/recaptcha_loader.test.ts --ignore-path ../../.gitignore from packages/auth
• .\node_modules\.bin\prettier.cmd --check packages/auth/src/platform_browser/recaptcha/recaptcha_loader.ts packages/auth/src/platform_browser/recaptcha/recaptcha_loader.test.ts .changeset/fix-auth-recaptcha-language-tags.md
• corepack yarn --cwd packages/auth test:browser:unit -- --grep recaptcha_loader (10 tests passed)
• git diff --check

Fixes #8357

[changeset-bot]

🦋 Changeset detected

Latest commit: f3aa811

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@firebase/auth	Patch
@firebase/auth-compat	Patch
firebase	Patch

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[gemini-code-assist]

Code Review

This pull request removes the six-character length restriction for browser language tags in the reCAPTCHA loader, allowing for longer tags such as 'en-GB-oxendict'. The change includes an updated validation function, a new test case to verify the fix, and a changeset file. I have no feedback to provide.