fix: remove dangerous defaults from Python create_order/build_order

[realfishsam]

Fixes #466

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Removes dangerous default values from create_order and build_order in the Python SDK. Previously, side defaulted to "buy", type defaulted to "market", and amount defaulted to 0. These defaults were hazardous -- a user who forgot to specify side or type would silently execute a market buy order. Now all three parameters are keyword-only (enforced by the bare * separator) with no defaults, requiring explicit values.

Blast Radius

• sdks/python/pmxt/client.py -- create_order() and build_order() signatures

Findings

1. Breaking change: all three params (side, type, amount) are now required keyword-only. Existing code calling exchange.create_order(market_id, outcome_id, "buy", "limit", 10, 0.55) with positional args will break because the bare * forces everything after outcome_id to be keyword-only. This is intentional and correct for safety.
2. Merge conflict with PR 600. PR 600 renames type to order_type in the same functions. If PR 598 merges first, PR 600 will need rebasing. If PR 600 merges first, this PR will need rebasing. Both PRs modify the same lines. The desired end state should combine both changes: keyword-only order_type (not type) with no defaults.

PMXT Pipeline Check

• Field propagation: N/A
• OpenAPI sync: N/A
• Type safety: OK -- stronger parameter requirements

Semver Impact

major -- removes default values and adds keyword-only constraint; breaks callers relying on positional args or omitted parameters

Risk

Merge conflict with PR 600 is certain. Coordinate merge order. No tests are added in this PR to verify the new required-keyword behavior.