Make sure duration parsing/date modification works with DateTimes

[fbacall]

Fixes #4
Fixes #5

[Copilot]

🟡 Changes recommended

The new duration parsing currently coerces all duration components to floats, which can lead to surprising numeric coercions/precision in date arithmetic and should be corrected for predictable behavior.

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Pull request overview

This PR updates duration parsing and date modification logic so ISO8601 durations that include only a time component (e.g., PT1H) are accepted, and so modify_date preserves time information when the start value is a datetime.

Changes:

• Update modify_date to parse full ISO8601 durations including PT... time-only values and to use DateTime when the input date string includes a time component.
• Add a regression test ensuring end datetime is correctly inferred from a start datetime plus a time-only duration (PT8H).

File summaries

File	Description
test/field_test.rb	Adds coverage for inferring an end datetime from a start datetime and an ISO8601 PT... duration.
lib/tess/rdf/extraction.rb	Reworks modify_date duration parsing and applies calendar vs fixed-unit arithmetic, preserving time when appropriate.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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