chore: rebrand sales agent as Prebid Sales Agent in md and comments

[patmmccann]

Motivation

• The codebase was rebranded to use the "Prebid Sales Agent" name and the A2A agent card now exposes that name, which caused e2e assertions to fail.
• Update tests to align expectations with the new runtime branding so agent card unit/e2e checks reflect current behavior.

Description

• Updated tests/e2e/test_a2a_endpoints_working.py to expect "Prebid Sales Agent" for the /.well-known/agent.json, /agent.json, direct create_agent_card() checks, and serialized agent-card roundtrip.
• No runtime behavior or production strings were changed in this patch; this commit only updates test assertions to match the previously-applied rebranding.

Testing

• Ran uv run pytest tests/unit/ -x which completed successfully: 1475 passed, 31 skipped.
• Ran uv run pytest tests/e2e/test_a2a_endpoints_working.py -k "agent_card" which completed successfully: 4 passed.

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Codex Task