Remove customizable separators; improve resource separator

[jlowin]

This is technically a breaking change because it removes user-customizable separators when mounting servers under a prefix. However, these customizable prefixes caused a problem for resources in particular because the resource prefix became part of the URI protocol, which has even more restrictions than other parts of the URI (like the path). For example, if you mounted a server with the prefix "sub_server" it would fail because protocols can't have underscores.

This PR modifies resource prefixes to become part of the path not the protocol. In other words, protocol://path/to/resource becomes protocol://prefix/path/to/resource in the new setup, which is far more natural. On the plus side this improves our support for arbitrary prefix mounts.

As an extension, customziable separators are no longer supported and will give a deprecation warning.

[Copilot]

Pull Request Overview

This PR removes support for custom separators when mounting or importing servers and updates resource prefixes to be part of the path rather than the protocol.

• Moves resource prefix handling into helper functions (add_resource_prefix, remove_resource_prefix, has_resource_prefix)
• Deprecates *_separator parameters with warnings and removes validation via Pydantic
• Updates tests, docs, and mounting/import logic to use the new protocol://prefix/path format

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/server/test_server.py	Adds parametrized tests for new resource prefix helpers
tests/server/test_openapi.py	Updates expected prefixed URIs in OpenAPI tests
tests/server/test_mount.py	Removes custom separator usage and updates assertions
tests/server/test_import_server.py	Updates import tests to new prefix format
tests/deprecated/test_mount_separators.py	Adds deprecation warning tests for separator args
tests/deprecated/test_deprecated.py	Mirrors deprecation tests for mount/import
src/fastmcp/server/server.py	Implements add/remove/has_resource_prefix, deprecates separators, updates mount/import logic
docs/servers/composition.mdx	Updates documentation to reflect new prefix format and removes custom separator section

Comments suppressed due to low confidence (1)

src/fastmcp/server/server.py:1315

• [nitpick] The variable name match shadows the built-in re.match function's return type. Consider renaming it to m or match_obj for clarity and to avoid confusion.

match = re.match(r"^([^:]+://)(.*?)$", uri)