Remove Sentinel workaround for Click 8.3.1+

[Copilot]

Click issue #3069 caused optional parameters to return Sentinel objects instead of None. This was fixed in Click 8.3.1, making the workaround obsolete.

Changes

• Removed _value_or_none() helper function that checked for Sentinel objects
• Removed all calls to _value_or_none() (11 occurrences across insert_upsert_implementation(), memory(), and _maybe_register_functions())
• Updated Click dependency to >=8.3.1 in pyproject.toml

The removed workaround was converting parameters like this:

# Before
def insert_upsert_implementation(..., convert, delimiter, encoding, ...):
    convert = _value_or_none(convert)      # Check if Sentinel, return None
    delimiter = _value_or_none(delimiter)  # Check if Sentinel, return None
    encoding = _value_or_none(encoding)    # Check if Sentinel, return None
    # ...

# After  
def insert_upsert_implementation(..., convert, delimiter, encoding, ...):
    # Click 8.3.1+ returns None directly for unset optional parameters
    # ...

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Remove these Sentinel hacks when Click 8.3.1 ships</issue_title>
<issue_description>This commit: 1361ed5

Has a workaround for this issue:

• Sentinel is passed to type_cast_value pallets/click#3069

The fix for that is set to go out in Click 8.3.1 - release issue here:

• Release 8.3.1 pallets/click#3087

Once that is released, remove the nasty hacks.

</issue_description>

Comments on the Issue (you are @copilot in this section)

@simonw This looks like it may be a clean revert:

git revert 1361ed571122ffd4dd8c3f1fdea52a1423422c04</body></comment_new>

• Fixes Remove these Sentinel hacks when Click 8.3.1 ships #666

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.33%. Comparing base (ae58faf) to head (20fa91a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #685      +/-   ##
==========================================
+ Coverage   95.32%   95.33%   +0.01%     
==========================================
  Files           8        8              
  Lines        3014     3001      -13     
==========================================
- Hits         2873     2861      -12     
+ Misses        141      140       -1     

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