cloud_storage_clients: make s3 url style optional

[nvartolomei]

Allow url_style to be explicitly unset (std::nullopt) to indicate that self-configuration needs to determine the addressing style, rather than implicitly defaulting to virtual_host. This makes the self-configuration logic more explicit about whether the URL style has been configured.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

• none

[Copilot]

Pull request overview

This PR modifies the S3 client configuration to make the URL style explicitly optional rather than implicitly defaulting to virtual_host. When url_style is std::nullopt, it indicates that self-configuration should determine the appropriate addressing style (virtual-host or path-style).

Changes:

• Changed s3_url_style url_style from a default value to std::optional<s3_url_style> initialized to std::nullopt
• Updated self-configuration logic to explicitly set the URL style instead of relying on implicit defaults
• Added guards to handle the optional URL style in various locations

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/v/cloud_storage_clients/configuration.h	Changed url_style from a default value to std::optional with std::nullopt default
src/v/cloud_storage_clients/s3_client.h	Changed _ap_style member to std::optional<s3_url_style>
src/v/cloud_storage_clients/s3_client.cc	Updated self-configuration logic to explicitly set URL style and handle optional values
src/v/redpanda/tests/fixture.cc	Added check for nullopt and default to virtual_host when converting config
src/v/cloud_io/tests/s3_imposter.cc	Added guard to handle optional url_style in test fixture