Raise config error on duplicate process_name within a group

[bardia-key]

Previously, when a [group:x] section combined two or more programs whose process_name expressions expanded to the same value (most commonly when numprocs > 1 is used and process_name does not include %(program_name)s), ProcessGroupBase.init silently dropped the collision: processes for a group are stored in a dict keyed by their expanded process name, so the later program's process configs simply overwrote the earlier program's entries in that dict. The result was that one of the programs never started, with no warning or error logged anywhere, making the failure very difficult to diagnose.

For example:

[group:a]
programs=b,c

[program:b]
process_name=%(process_num)02d
numprocs=10

[program:c]
process_name=%(process_num)02d
numprocs=10

expected 20 total processes but only started 10, because programs b and c both expand process_name to "00".. "09" and c's entries silently replaced b's.

This commit adds duplicate-name detection to
process_groups_from_parser() while building a heterogeneous group's process list: as each program's processes are added, we track which program produced each expanded process_name, and raise a ValueError identifying the colliding process name and both programs as soon as a collision is found. This matches the existing convention in this file of raising hard config-parse errors for other invalid process_name/ numprocs combinations (e.g. missing %(process_num)s) rather than allowing supervisord to start in a broken, partially-functioning state.

Also updates documentation and the changelog:

• docs/configuration.rst: adds a note under the process_name option explaining the group-wide uniqueness requirement and the new startup error, and a matching note under [group:x]'s programs option, cross-referenced via a new groupx_section label.
• CHANGES.rst: adds an entry under 4.4.0.dev0 describing the bug and the fix.

Adds three tests to supervisor/tests/test_options.py:

• test_duplicate_process_name_in_heterogeneous_group: reproduces the numprocs/process_name collision above and asserts the ValueError mentions the colliding name and both program names.
• test_duplicate_process_name_in_heterogeneous_group_default_process_name: covers a collision via identical fixed process_name strings (no numprocs involved).
• test_no_duplicate_process_name_error_when_names_differ: regression guard confirming process_name expressions that differ per program (e.g. via %(program_name)s) still produce the full expected process count with no error.