Add unit tests for IndexNowPathProcessor and IndexNowOutboundPathProcessor

Problem/Motivation

IndexNowPathProcessor and IndexNowOutboundPathProcessor
have no dedicated unit tests. Both are only exercised indirectly through kernel
tests, which leaves their core logic unverified in isolation:

• IndexNowPathProcessor::processInbound() rewrites the public
  /{key}.txt path to the internal
  /index_now_api_key/{key} route. The guard conditions (empty
  key, path mismatch) are never explicitly asserted.
• IndexNowOutboundPathProcessor::processOutbound() overrides
  $options['base_url'] when the index_now option
  and the index_now.base_url setting are both present. The
  combinations where one condition is missing are not tested.

Proposed resolution

Add tests/src/Unit/PathProcessor/IndexNowPathProcessorTest.php
covering:

• Path is rewritten when the key matches.
• Path is returned unchanged when the key is empty.
• Path is returned unchanged when the path does not match the key.

Add tests/src/Unit/PathProcessor/IndexNowOutboundPathProcessorTest.php
covering:

• base_url is overridden when both the index_now
  option and the index_now.base_url setting are set.
• Trailing slash is stripped from the configured base URL.
• base_url is left unchanged when the index_now
  option is absent.
• base_url is left unchanged when absolute is not
  set.
• base_url is left unchanged when index_now.base_url
  is not configured.

All dependencies (IndexNowKeyManagerInterface,
Settings) should be mocked or stubbed. No Drupal bootstrap is
required.

Remaining tasks

• [ ] Write IndexNowPathProcessorTest.php
• [ ] Write IndexNowOutboundPathProcessorTest.php
• [ ] Ensure all scenarios listed above pass

User interface changes

None.

API changes

None.

Data model changes

None.