routerrpc: fix fee limit validation and calculation

[asmit27rai]

Change Description

This PR fixes inconsistent behavior in fee limit validation and calculation for both SendPayment and SendPaymentV2 RPC calls. The changes:

1. Make fixed and fixed_msat fee limits mutually exclusive (error if both set)
2. Properly handle percentage-based fee limits in combination with fixed limits
3. Add validation for fee limit fields
4. Maintain backward compatibility with existing behavior
5. Add helper functions for consistent fee limit calculation

Fixes: #7832

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[asmit27rai]

@saubyk @AndySchroder
Pls review this PR..

[saubyk]

Hi @asmit27rai please go through the guidelines published for new contributors:
https://github.com/lightningnetwork/lnd/blob/master/docs/review.md

New contributors are expected to contribute to code review first, before expecting review on their prs. Thanks.