feat(prices): validate latest_value_only vs dates, widen default date range

[Bloom79]

Description

Three small related improvements to MarketPricesApi.get_prices(), coming from the review comments in #36:

• Validation (Does latest_value_only mean start and end date are ignored? If so, I suggest adding a validation that raises an error if use sets both latest_value_only and the dates. #42): latest_value_only=True combined with start_date/end_date now raises a ValueError instead of silently ignoring the dates.
• Better default (Default values for start_date is the same as for end_date, creating a time range of length 0. Is that intended? Could a more helpful default start_date be today - N(7?) days? #39): when no start_date is given it now defaults to 7 days before today (constant DEFAULT_DATE_RANGE_DAYS) instead of today, avoiding a zero-length default date range. Happy to change N if you prefer a different window.
• Constant (Suggest creating a constant for "%Y-%m-%dT%H:%M:%S+01:00" and using throughout to avoid typo bugs #40): the repeated "%Y-%m-%dT%H:%M:%S+01:00" literal is extracted into RFC3339_DATE_FORMAT and used for all date formatting.

Added tests/unit/test_market_prices.py (6 tests, pure mocks — no API credentials needed) covering the validation, the defaults and the RFC 3339 formatting. Docstring updated (new default, Raises section).

Developed with AI assistance (Claude Code); all changes reviewed and tested locally (make codestyle clean, unit suite green).

Related Issue

Closes #42
Closes #39
Closes #40

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've updated the code style using make codestyle.
• I've written tests for all new methods and classes that I created.
• I've written the docstring in Google format for all the methods and classes that I used.
• My commits follow the Conventional Commits format (fix:, feat:, chore:, etc.).