[19.0][FIX] server_environment: add search support for env-computed fields

[AungKoKoLin1997]

Summary

When server_environment transforms a field to an env-computed field, it sets store=False but does not provide a search method. This makes the field unusable in domain filters, e.g.:

storage_id = fields.Many2one(
    comodel_name="storage.backend",
    domain=[("backend_type", "=", "sftp")],
)

raises:

ValueError: Cannot convert storage.backend.backend_type to SQL because it is not stored

This was not an issue before Odoo 19 because the old domain processing (osv.expression) handled non-stored fields more leniently. Odoo 19's rewritten domain engine (odoo.orm.domains) strictly requires either store=True or an explicit search method.

Fix

Add a search method for env-computed fields following the same _partialmethod pattern already used for the inverse method.

@qrtl QT7028