Preserve custom query parameters in index search forms

[BriceFab]

Fixes #7640

Summary

Browsers discard the query string of GET forms when they are submitted. EasyAdmin already works around this for:

• official filters[...] parameters (hidden fields in search_form_filters, see [3.5.9] Searching does not respect active filters #4707)
• the filters modal form (form_action_query_string_as_array|ea_flatten_array in filters.html.twig)

However, other query parameters (custom menu context like _status, sort state, etc.) are preserved in links generated by AdminUrlGenerator, but lost when submitting the index search form in real browsers. This has been known since #1573.

This PR generalizes the existing approach by preserving all remaining query parameters as hidden fields in the search form.

Changes

• Add SearchDto::getPreservedQueryParameters() to return current query params minus reserved EasyAdmin keys (query, page, filters, batch params, etc.)
• Render those parameters as hidden fields in layout.html.twig (same comment/pattern as filters.html.twig)
• Add unit and functional tests

Test plan

• SearchDtoTest::testGetPreservedQueryParameters
• SearchAllTermsTest::testSearchPreservesCustomQueryParameters

Related issues

• How to pass new url parameter in entity in Easy Admin Bundle? #1573 (search button does not pass custom parameters)
• [3.5.9] Searching does not respect active filters #4707 (search did not respect active filters — fixed for native filters only)