Validate futures series contract dates

[JustinMShea]

Adds validation for futures contract suffixes and dates. Invalid dates now produce clear errors, inconsistent contract months produce warnings, and the existing continuous-contract and spread formats remain supported. Adds focused tinytest coverage without changing the public API.

[Copilot]

Pull request overview

This PR adds internal validation for futures series identifiers and contract dates, surfacing invalid inputs as clear errors and warning when an outright suffix month/year conflicts with the supplied expiration, while keeping existing storage formats (including YYYY-MM) and series/spread/continuous formats supported.

Changes:

• Introduces internal parsing/validation helpers for suffixes and date inputs, including mismatch warnings and invalid-date errors.
• Wires the new validation into future_series() construction flow.
• Adds focused tinytest coverage for successful creation, warnings, and error cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
R/instrument.R	Calls the new internal validator during future_series() construction.
R/future_series_validation.R	Adds internal helpers to parse/validate suffixes and (month/day) date inputs, emitting warnings/errors.
inst/tinytest/test_future_series.R	Adds regression tests covering valid series creation, mismatch warnings, and invalid inputs.

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