Return new instances of big int for DefaultConfig

[facuMH]

This PR fixes https://github.com/0xsoniclabs/sonic-admin/issues/687

The Problem:
A data race was detected in integration tests when multiple nodes attempted to load or save their configurations concurrently. This occurred during the TOML marshaling/unmarshaling process.

To reproduce, checkout commit c89694f and run the following command:

go test ./tests/ -race -count 1 -run TestIntegrationTestNet_CanDefineValidatorsStakes

The resulting race detector log points to UnmarshalTOML for both the read and write operations.

The Root of the issue:
The DefaultMaxGasPrice global variable was defined as a *big.Int. When the default configuration was copied to load/save the node config, only the pointer address was copied, not the underlying value.

When an integration test spins up multiple nodes simultaneously:

• Both nodes reference the same memory address for their max gas price.
• During UnmarshalTOML, the *big.Int value is modified in place.
• Concurrent read/write access to this shared pointer without synchronization triggers the data race.

The Solution
Convert the global variable DefaultMaxGasPrice into a function that returns a new *big.Int instance on every call.

[Copilot]

Pull request overview

Fixes a race condition during concurrent TOML marshal/unmarshal of node configuration by ensuring the default max gas price is not shared via a global *big.Int pointer.

Changes:

• Replaced the exported DefaultMaxGasPrice global *big.Int with a function that returns a new *big.Int per call.
• Updated default config/oracle initialization to call DefaultMaxGasPrice() instead of referencing a shared pointer.

Reviewed changes

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

File	Description
gossip/gasprice/gasprice.go	Converts default max gas price from shared global pointer to per-call allocation; updates oracle initialization accordingly.
gossip/config.go	Updates default gossip config to use gasprice.DefaultMaxGasPrice() when setting GPO.MaxGasPrice.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
gossip/config.go	88.80% <100.00%> (ø)
gossip/gasprice/gasprice.go	83.82% <100.00%> (+0.49%)	⬆️

... and 11 files with indirect coverage changes

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[LuisPH3]

The PR identifies the source of races during testing, this issue cannot happen in production, but shadows mechanism to detect other race conditions.

• The cause was rather old. Weird that it did not happen before.
• The change introduces regression testing