refactor: Align nonce computation with other mobile SDKs

[NandanPrabhu]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

Nonce computation is refactored to match other SDKs in following steps

• by the value user passes through func nonce(_ nonce: String) -> Self of WebAuth
• by the value user passes through parameters func parameters(_ parameters: [String: String]) -> Self
• if value is not passed through either ways. We compute default value for nonce same way state is computed
• Now nonce will always be non optional value and it is ensure different authorization request from same Auth0WebAuth instance will always have different nonce and state values

📎 References

🎯 Testing

Unit tests updated to add coverage for code for nonce computation refactoring

[Copilot]

Pull request overview

This PR updates Auth0WebAuth to persist and propagate state/nonce values through the web auth flow (including authorize URL construction and ID token validation), and adjusts the test suite accordingly.

Changes:

• Generate and store default state and nonce in Auth0WebAuth.parameters when missing.
• Thread nonce through start() into buildAuthorizeURL(...), and pass it into the PKCE handler for ID token validation.
• Update WebAuthSpec to exercise the new nonce/state behavior and to pass a nonce into buildAuthorizeURL(...) in URL-building tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
Auth0/Auth0WebAuth.swift	Adds cached generation for state/nonce, threads nonce into authorize URL building, and uses nonce for PKCE/ID token validation.
Auth0Tests/WebAuthSpec.swift	Updates authorize URL tests and adds/adjusts expectations around state/nonce generation and persistence.

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[sanchitmehtagit]

LGTM, Could we please add context/background too on PR description and better PR title

[sanchitmehtagit]

1 NIT