fix(ai): stop percent-encoding slashes in ai.run() model_name

[nileshpatil6]

Fixes #2718

client.ai.run() builds its request path with path_template(), which percent-encodes every character in a path placeholder, including forward slashes. Model names like @cf/meta/llama-3.1-8b-instruct end up in the URL as @cf%2Fmeta%2Fllama-3.1-8b-instruct, and the API responds with a 400 "No route for that URI". This broke every ai.run() call after the v5 rewrite, since v4.3.1 did not encode slashes this way.

Root cause is in src/cloudflare/_utils/_path.py. path_template() treats / as unsafe in the path portion (correct for a normal single path segment, since it protects against path traversal and segment confusion), but model_name is not a single path segment, it is a slash separated identifier that the Cloudflare API expects literally in the URL.

Fix: added support for RFC 6570 reserved expansion syntax in path_template. A placeholder written as {+name} keeps / unescaped in the interpolated value while still percent-encoding every other unsafe character, and the existing dot-segment rejection (. / ..) still runs on the fully assembled path, so this does not weaken the path traversal protection that was added for the normal {name} syntax. Updated the ai.run() path template (both sync and async) to use {+model_name} instead of {model_name}.

I picked the reserved-expansion approach instead of a one-off hack in ai.py because this is a general problem: any path parameter that is itself a multi-segment identifier (this one, and potentially others in the future) needs the same treatment, and RFC 6570 already has a standard operator for exactly this.

Testing:

• Added test cases to tests/test_utils/test_path.py covering: the reported model_name case, that other special characters (space, ?) are still escaped under {+name}, that the same variable used elsewhere in the same template without + is unaffected, and that dot-segment rejection still fires for {+name}.
• Ran the full test_path.py suite locally, 56 passed.
• Ran ruff check and ruff format --diff on the changed files, both clean.
• Manually reproduced the bug against the module directly before the fix (got @cf%2Fblack-forest-labs%2Fflux-1-schnell) and confirmed the fix produces @cf/black-forest-labs/flux-1-schnell.