test(openai-agents-v2): improve test coverage

[nagkumar91]

Description

This PR improves test coverage for the opentelemetry-instrumentation-openai-agents-v2 package by adding 66 new unit tests.

Coverage Improvements

File	Before	After	Change
__init__.py	82%	99%	+17%
span_processor.py	77%	81%	+4%
Overall	78%	83%	+5%

New Test Coverage

The new tests cover previously untested code paths:

Helper Functions (__init__.py):

• _resolve_system - System name resolution with enum matching
• _resolve_bool - Boolean parsing from various input types
• _resolve_content_mode - Content capture mode resolution

Utilities (span_processor.py):

• normalize_provider - Provider name normalization
• validate_tool_type - Tool type validation
• normalize_output_type - Output type normalization
• ContentCaptureMode properties (capture_in_span, capture_in_event)
• _get_span_name for handoff/transcription/speech span types
• _infer_output_type for function/embeddings/image/audio/json outputs
• _sanitize_usage_payload for dict and object usage payloads
• _normalize_messages_to_role_parts edge cases
• _normalize_to_text_parts content normalization
• _collect_system_instructions message collection
• Processor configuration options (metrics disabled, custom system name)

Implementation Notes

• Uses lazy imports within test functions to avoid module caching conflicts with existing tests
• All 91 tests pass (25 original + 66 new)
• No changes to existing test files or production code

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change improves tests
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• Followed the style guidelines of this project
• Unit tests pass locally with my changes
• Changes generate no new warnings

[rads-1996]

LGTM