Fix get_parameter_source() returning None in type.convert() and callbacks

[Priyanshu-byte-coder]

Summary

Fixes #3458

In 8.4.0, handle_parse_result() was refactored to support feature-switch group arbitration (#3403). As part of that refactor, ctx.set_parameter_source() was moved to after process_value() completes. This broke any ParamType.convert() implementation or option callback that calls ctx.get_parameter_source(param.name) — they received None instead of the actual source.

This also causes pallets/flask#6025 (FLASK_DEBUG env var stopped working in click 8.4), where Flask's _set_debug callback relies on get_parameter_source() to distinguish default values from user-set ones.

Root cause

consume_value()              ← source determined here
process_value()              ← type.convert() / callback called here
  └─ type.convert(value, param, ctx)
       └─ ctx.get_parameter_source(param.name)  ← returns None! ❌
ctx.set_parameter_source()  ← source registered here (too late)

Fix

Set the source immediately after consume_value(), before process_value() is called. If the parameter subsequently loses feature-switch arbitration (another parameter with a more explicit source already claimed the slot), restore the winner's source.

consume_value()              ← source determined here
ctx.set_parameter_source()  ← registered early ✅
process_value()
  └─ type.convert(value, param, ctx)
       └─ ctx.get_parameter_source(param.name)  ← returns correct source ✅
arbitration                 ← restore winner's source if we lose

The arbitration semantics for ctx.params are unchanged.

Test plan

• test_get_parameter_source_available_in_type_convert — asserts convert() sees DEFAULT for default invocation and COMMANDLINE for CLI invocation
• test_get_parameter_source_available_in_callback — same check for option callbacks
• All existing test_parameter_source parametrized cases still pass
• Feature-switch group tests (if any) still pass