fix: handle tuples in deepcopy_minimal to prevent mutation of user data

[roli-lpci]

Summary

deepcopy_minimal copies dict and list but intentionally skips tuple (returning it as-is). When files.beta.upload() is called with a 4-element FileTypes tuple containing a headers dict, the tuple passes through uncopied. httpx then mutates the user's original headers dict by injecting Content-Type during multipart encoding.

headers = {'X-Custom': 'value'}
file = ('test.txt', b'data', 'text/plain', headers)
client.files.beta.upload(file=file)
print(headers)  # {'X-Custom': 'value', 'Content-Type': 'text/plain'} — mutated by httpx

Users who reuse file tuples across multiple uploads see their headers silently contaminated.

Changes

• src/anthropic/_utils/_utils.py — Add tuple handling to deepcopy_minimal, matching the existing pattern for dicts and lists. Tuples are recursively copied so mutable elements (like headers dicts) get independent copies.
• tests/test_deepcopy.py — Replace the old assertion that tuples pass through uncopied (assert obj3 is obj4) with 4 new tests covering: simple tuples, tuples with mutable contents (the bug scenario), dicts containing tuples (files.beta.upload pattern), and lists of tuples (skills/versions pattern).

Note on behavioral change: The previous test explicitly asserted tuples are NOT copied. This was a deliberate scope limitation of deepcopy_minimal that did not anticipate the httpx mutation path. This PR intentionally reverses that — tuples are now recursively copied like dicts and lists.

Note on Stainless: _utils/_utils.py has no codegen header, consistent with hand-maintained utility code. If this change should be applied to the generation template instead, happy to redirect.

Testing

• 550 existing tests pass, 0 regressions
• 4 new tests verify tuple copying and mutation isolation

Fixes #1202