Add test coverage for parse_edid

[Copilot]

parse_edid in tools.py had no test coverage. This PR adds a _make_edid helper and 12 tests covering all major code paths.

Tests added

• Error paths: too-short blocks, invalid checksum, invalid header
• Basic parsing: id_legacy field construction from packed manufacturer ID + product code
• Manufacture date: year-only, week+year, model year (manufacture_week=0xFF)
• Serial number: integer form, not set, string descriptor form, string overriding integer
• Descriptors: serial number string (0xFF), display name (0xFC)

Helper

_make_edid() constructs a valid 128-byte EDID block with correct checksum, supporting all fields needed by the tests:

_make_edid(
    manufacturer="TST",
    product_code=0x1234,
    serial_number=99,
    manufacture_week=10,
    manufacture_year=30,           # 1990 + 30 = 2020
    descriptors=[(0x48, 0xFF, "STRSERIAL")],
)

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